Properly object oriented retained mode GUIs let you subclass components to implement custom views and event handling, and develop and publish reusable prefabs and libraries of pre-configured, stylized, specialized component.
But immediate mode GUI lets you compose very well. It's a bottoms up technique of programming, where instead of following a framework established by the library author, you are given low level building blocks. You will find it hard to mix and match two retained mode GUI libraries unless they all follow a common set of standard apis (e.g., java swing). But with immediate mode GUI, it's actually easy to mix and match (as the immedate mode gui has no need to know what framework you're working under nor need to make any assumptions about threading or api).
I've implemented various adaptor wrappers for composing different retained mode API's myself, including wrapping The NeWS Toolkit OPEN LOOK widgets in HyperLook widgets, which I used for the first Unix port of SimCity to HyperLook/NeWS. (The OPEN LOOK pin-up menus and pie menus and buttons and sliders are implemented in TNT, but I wrapped them for HyperLook (which has its own persistence system and class hierarchy), so you can dynamically create and edit them at runtime with property sheets and script editors, ala HyperCard.)
https://medium.com/@donhopkins/hyperlook-nee-hypernews-nee-g...
For X11/NeWS at Sun, we even made an ICCCM X11 window manager that wrapped X11 windows in NeWS tabbed window frames with pie menus! (The X11 window manager was written in object oriented "retained mode" PostScript code, subclassing standard and custom NeWS window and menu widgets, like tabbed window frames and pie menus. While PostScript has an immediate mode drawing API, it was driven by extensible "retained mode" widget instances and classes represented by PostScript dictionaries, pushed on the dict stack.)
Open Window Manager PostScript source: https://www.donhopkins.com/home/archive/NeWS/owm.ps.txt
Tabbed window extension: https://donhopkins.com/home/archive/NeWS/win/tab.ps.txt
Discussion of problems with NeWS: https://news.ycombinator.com/item?id=15327339
Political discussion from the Window System Wars, about Sun's problems merging X11 and NeWS: https://donhopkins.com/home/archive/NeWS/sevans.txt
More recently (and more successfully), I've wrapped HTML components and floated them over OpenLaszlo/Flash (not so recently) and Unity3D (quite recently) retained mode user interface layouts in the web browser.
For example, I'm embedding an ACE code editor in a Unity3D retained mode user interface running in WebGL, that falls back to a regular TextMeshPro input field on other builds. There's no way you could do that practically with an immediate mode API.
WebGLAce_TMP_InputField.cs (C# retained mode GUI TMP_InputField subclass): https://github.com/SimHacker/UnityJS/blob/master/Libraries/W...
WebGLAce.cs (C# P/Invoke Unity extension wrapper): https://github.com/SimHacker/UnityJS/blob/master/Libraries/W...
WebGLAce.jslib (JavaScript library Unity extension, compiled by Unity to WebAssembly, converts between C#/WebAssembly and JavaScript data types and thunks messages back and forth between the browser and the Unity3D app): https://github.com/SimHacker/UnityJS/blob/master/Libraries/W...
WebGLAce.jss (ACE code editor adaptor, pure JS source loaded by the browser, as well as ACE JS libraries themselves): https://github.com/SimHacker/UnityJS/blob/master/Libraries/W...
Notice how WebGLAce_TMP_InputField simply subclasses the standard TextMeshPro TMP_InputField, and adds some more hidden state like the editorID and editorConfigScript of the corresponding ACE code editor running in the web browser.
All that is hidden from the code using it, so it doesn't need to come up with another place to store that editorID and editorConfigScript, or even know if it's getting an Ace editor under WebGL or falling back to TextMeshPro on other builds. That's what I mean by leveraging OOP to subclass existing components and hide internal state, which you can't do in immediate mode.
(My plan for non-WebGL builds that aren't already running inside a web browser, is to create an embedded web browser just to run the Ace code editor, and the C# WebGLAce_TMP_InputField API will remain the same, and it will just work transparently on different platforms. I want to support live coding Unity apps in JavaScript, but there just isn't anything for Unity that approaches ACE for editing code, so it's worth going through all the trouble to make an adaptor.)
How would you write a functional extension for Unity3D's old immediate mode GUI that let you embed an ACE code editor in a Unity WebGL app? What would the API and state management even look like? How could you make it platform independent?
And even if you solved all of those problems with an immediate mode API, by its very nature it still wouldn't enable you to build GUIs in the interface editor or store them in prefabs, and you'd still have to recompile you app (which can take half an hour with Unity) every time you wanted to tweak the user interface (which is why I like programming Unity apps in JavaScript as much as possible).
UnityJS Unity3D / JavaScript bridge architecture: https://github.com/SimHacker/UnityJS/blob/master/doc/Anatomy...
UnityJS pie menu component for Unity3D, written in JavaScript, drawn with canvas, using TextMesh Pro labels: https://github.com/SimHacker/UnityJS/blob/master/Libraries/U...
Uses a Unity3D pie menu tracker to translate low level input to high level events, written in C#: https://github.com/SimHacker/UnityJS/blob/master/Libraries/U...
Of course you can take it too far, trying to please everyone by combining the worst of all possible systems and then giving it a ridiculous sounding name (see "MoOLIT"), but we're discussing whether it's possible and practical, not whether it's desirable. And even if it's not desirable, it ends up being necessary.
https://en.wikipedia.org/wiki/MoOLIT
http://nova.polymtl.ca/~coyote/open-look/01-general/faq-doc-...
Embedding object oriented "retained mode" widgets with different APIs inside of each other is old hat and common for backwards compatibility. Whenever you write a new GUI toolkit, embedding widgets from the last toolkit is one of the first things you do (including recursively embedding widgets from the toolkit-before-last).
Concrete example: Microsoft lets you embed old fashioned OLE controls in Windows Forms / Presentation Foundation applications, which might be implemented in MFC themselves. And MFC is all about embedding old Win32 widgets implemented in C, and newer OLE components implemented in Visual Basic or whatever, in venerable C++ MFC user interfaces. Say what you want about how much Win32/MFC/OLE/WF/WPF sucks, and I'll wholeheartedly agree, but if you're running Windows, you probably have widgets on your screen using several different retained mode APIs embedded more than two levels deep right now.
The problem with immediate mode that is you have to come up with somewhere to store and retrieve any values or state required on a case-by-case basis (including the edited value itself, and other view state like scrollbar state for text editors, etc), and that tightly couples your component function with whatever's using it, so they're not very reusable or simple. With OOP, the object has its own place to store that stuff, which is cleanly decoupled from whatever's using the component.
Then there's the issue of event handlers. Some user interface components just aren't so simple that they only have one true/false return value like buttons. Text editors can notify on value change, end edit, select, deselect, etc. And Unity's retained mode GUI supports persistent event handlers that let designers hook up events with methods of objects with parameters, without writing or recompiling any code.
And there's also a lot more to layout that you can easily express with an immediate mode GUI. Sometimes you have to measure a bunch of things and align them in various ways, like grids or flex layouts, and adapt to the screen size and other constraints (i.e. responsive design), and that's really hard to do with immediate mode, while retain mode has a rich set of layout components you can use and extend. And there's nothing like XCode's layout constraints for immediate mode.
A perfect example is TextMesh Pro's text editor. It has so many different (and useful) features and parameters and callbacks and ways to configure it, and it keeps so much state in order to redisplay efficiently with the fewest number of draw calls and reformatting, that it would be extremely impractical and inefficient and unwieldy for it to use an immediate mode API. Especially with C# positional arguments instead of Pythonic or Lisp-like optional unordered keyword arguments.
When you encapsulate your immediate mode layout in a class, or write an engine that interprets (i.e. JSON) data to drive the immediate mode API, or use reflection in your wrapper functions to dispatch event callbacks and bind your widget data to your model data, you're just rolling your own informally-specified, bug-ridden, slow implementation of half of retained mode. (See Greenspun's tenth rule.)