Tk based GUIs work across pretty much all desktop platforms and are much less resource intensive than electron and friends.
In my (limited) experience, Tcl/Tk is great for basic stuff but I don't think you could do even a small part of what you can do with an html + css + javascript GUI.
Say you wanted to write a CAD app like https://www.onshape.com. There’s no way you could do it Tcl/Tk, is there?
2. I have seen very basic GUIs in electron just for the cross-platform nature of it.