>"I don't get everyone's complaints about the interface. I've been using it for years..."
That's the problem, you have a bias because you've already learned the tribal knowledge. Having to learn it from scratch, GIMP's UI is sorely lacking. While it does have the tools, the organization of them is all over the place.
I have used Photosohop for almost 20 years, and then Affinity, Krita, and Gimp here and there. In my opinion, it's a bit of both. At first I thought I was just used to Photoshop and I had to just accept losing productivity to make the switch. But there something about Gimp UX that I can't get over, even though I can't really explain what is it. Of course fighting the muscle memory for all the features is a pain, but there's more. I can't find logic in the shortcuts, the menu organisation... I have not struggled as much for Krita or Affinity when I tried it.
Are you sure that the problem isn't just that you have learned a different tribal knowledge (e.g. Photoshop) and now expect every image application to work that way?