For an expert with some mathematical intuition layout constraints can do a custom layout that lays out perfectly on all screen sizes much better than declarative. However there is a learning curve I admit.
Apple’s documentation has also gotten way, way worse. Apple expects you to watch the WWDC videos, but it’s not like they make refresher videos on old topics all the frequently and they often remove or hide old videos.
And yes StackView is your biggest friend. Usually when the system ignores your constraint it says that on the log. It can be a bit tricky to find out which constraint its breaking but the View Debugger and also the constraint constant value can help find it.
Eventually I tried going back to manual layout because a coworker still used it all the time and I noticed that it was pretty easy to follow the layout rules in imperative code (so long as it's well-structured). Since then I just do manual layout for everything. It's just easier for my brain to describe the layout I want that way. I feel like it takes me longer to come up with constraints that match what I want, but I'll admit if I went back to auto-layout, I'd probably be better at it.