It's really a shame. As an example, it's been more than a decade and they still haven't fixed the utterly absurd "layer boundaries" system that it has. There's a reason no other image editor does that, and it's because it makes no sense.
(To elaborate, every layer in GIMP has a defined "boundary rectangle" which cannot be drawn outside of.)
Maybe this is a good example to talk about good and bad UI.
There are UIs that are easy to learn, but less efficient in daily use.
There are UIs that are difficult to learn, but efficient in daily use by experienced users.
Good UIs steer an appropriate course between those options, according to the needs of their users.
GIMP's "boundary rectangle" system provides a learning barrier for new users, and a regular annoyance for experienced users. If people don't like it, the problem isn't that they're Adobe shills, the problem is that it's indefensiblybad.
OK I guess it saves some ram if you need to run GIMP on a Raspberry Pi, but this is not a sensible thing for GIMP to be optimizing for.
I fully accept that it's run by volunteers and it's OSS and I could patch it myself etc. I don't blame the volunteer devs for not making a perfect program in their spare time. But I wish people would lay off blaming discontent with GIMP's UI on the unsuitability of its users or the existence of Photoshop.
I guess I'm in the minority, but not only does this behavior not bother me, for me it's a feature.
Most of the time, if I'm trying to draw outside the boundary, then it's because I've got the wrong layer selected. So it saves me from drawing on the wrong layer and then having to deal with trying to get the changes moved to the correct layer.
In cases where the above doesn't hold (i.e., I actually want to draw inside the smaller layer), then it's easy enough to expand the layer's size.
What do you expect to happen if you do a flood fill when there isn't a boundary rectangle? (legitimate question, I don't use any of these tools)
My assumption is that there is at least an image boundary, and just GIMP allows layers to have a different boundary to the image. But if I didn't allow this, then every time I moved a layer, it would get cropped to the image boundary.
Non-destructive editing is overrated imho. If a user has been saving files all along reverting back is easy. They can even import certain layers if it's only a 'partial reversion'.
There's more to Non-Destructive Editing than just a fancy 'undo'. It allows you to go back later and adjust/refine/tweak the alterations you've made to each layer in your image, individually. You would need to be saving dozens of copies as you went along, covering every possible permutation [number of layers x adjustments per player] to allow you the same freedom to re-adjust that non-destructive editing provides.
Then, you'd have to keep all those multiple copies in case you ever needed to make adjustments to that image in the future. With non-destructive editing, you have one file which contains everything. It's not just a gimmick!
What annoys me the most about the interface is that dialogs ALWAYS open on the wrong screen. Is it so hard to open the dialogs on the same screen as the main window?
[Oh yes. The interface is awful, too]