They seem to want to differentiate themselves as (e.g. "not photoshop" in gimp's case) but seem to equate that with "ignoring good ui/ux design".
I've been using Photoshop for a long time, and I've learned a lot of its shortcuts and intricacies. Basically, when I want to accomplish something in Photoshop, I already have an idea on how I'll go about it, using the functionality that's available. But GIMP, on the other hand, never really clicked for me. I find it very unintuitive and limiting, and it's a huge pain to have to do something in GIMP when Photoshop's not readily available. I've convinced myself that this is because GIMP has a much inferior UX and is orders of magnitude more limiting than PS (at least the subset of their features I use in my day-to-day usage).
On the other hand, since my light vector editing needs have been satisfied by Photoshop for a long time, I haven't really learned Illustrator. Recently, for various reasons, I've had to do some heavier-than-usual vector editing stuff, but still nothing requiring more than simple Beziers, fills and strokes, so I've been doing it in Inkscape since it's just been handy. After some time, I decided to try and use Illustrator, figuring it'd be like a whole new world. And then, surprisingly, I realized I don't really like it. The interface was illogical and not in line with my mental model at all. I struggled to complete basic tasks, and finally gave up and did the job in Inkscape. Basically, it was very reminiscent of the Photoshop―GIMP situation.
So my conclusion is that the tools and their UX are very powerful in giving me a mental model of a task, and significantly more so than I would have imagined. So it might not be 100% true to say that the UX in these tools is inferior. It's just so different from what we're used to that we have a very, very hard time separating the "different" from "worse" in our heads.
The expected behaviour in Photoshop is that if you drag-move, you only move the selected layer. It seems like in GIMP if you drag-move, you drag the highest layer that has a painted pixel under your cursor. There are probably situations where this saves time, but more often I try and drag some text around and I end up moving a background layer by mistake.
I often find myself using Inkscape to save time, it's intuitive enough and it works well.
All in all, I've always found Gimp more intuitive and easy to use than Photoshop, probably because I learned it first!
It does take a few days for things to sink into your noggin after so long in a different tool, but I certainly think Inkscape's quality is very high. It's worth learning, unlike (imo) GIMP.
I don't think the Inkscape UI is really that hard to understand, it's just different. Granted, I haven't used Photoshop and Illustrator since version 6.0, but I had no trouble getting into Gimp or Inkscape within a couple of days of using it. I didn't find it hard to grok the UI and the intended behavior, it became pretty usable very quickly (bugs aside...).
As a developer, I can easily understand what the intended behavior in these programs is, everything is very logical.... but maybe that's the problem many users have ;)
Edit: and I can see how the bugs can confuse users to no end. It might be even the greatest disadvantage, especially inkscape behaved really buggy the last time I used it half a year ago :(
It's easy to understand the behavior and UI from a programmer/developer's perspective, but the main portion of the target group in this sector does think completely different.
For a thing that should probably take about an hour, it takes me half a day, to relearn the handful of things I need to do.
Which, I think, is pretty good, considering I'm not at all a graphics person, and usually have to search around just to discover the terms I need to search for how to do what I want.
My results are amateurish, but good enough, and probably wouldn't exist without Inkscape. I like it.
Inkscape's interface isn't too bad, not good, but I rarely have significant issues with it. GIMP is an absolute nightmare of UI/UX design, I can't understand how someone hasn't fixed that by now.
The issue, as you point out, is lack of designer. Both Qt and GTK can be extensively customised, but you need to know what you're doing (in an artistic sense).
Of course you can understand that. Try writing actual code to fix any pet peeve with GIMP's UI/UX (or any app's UI/UX). Then we'll sit down and talk again about the role of volunteers in a free software project :)
The bad thing about the UI at the moment for me is the lack of HiDPI support (as far as I can tell). And back when I was using OSX, the interface seemed a bit janky since it used XQuartz and it generally wasn't very native both in terms of UI rendering and its default keyboard shortcuts.
Nope, we bloody well know GIMP's UI sucks in many ways. But UI doesn't fix itself as you might suspect. For every few thousands of people who complain about its UI we maybe get just one occasional contributor.
Adobe Illustrator is, for a novice, surprisingly not that much better.