They have a lot of trouble creating tools that aren't patented by Adobe/ photoshop so they have to use loop holes to get them out. It often leads to the UI complaints people have. It's not really note worthy, but legally they can't copy and paste the structure of photoshop, even though they're a lot like them. So their software tends to be unintuitive. If that makes sense. It's mostly the tool names and the UI.
It's specifically the tools that were created by Adobe that take a while to get implemented into GIMP. If someone makes a tool in a FOSS program, it's an open playing field. Sadly, they're mostly copying Adobe/ photoshop so they run into potential legal issues.
Gimp also struggles with layer/image fidelity, but that's more of a resources thing.
But in all seriousness, I am super excited for the future of Gimp. I feel like 3.0 going to be a turning point for rhe software, sort of like Blender had with 2.8.
I used Gimp about 15 years ago and I reached out to a guy who worked on Attack of the Show asking him for any tips on how to use Gimp. Surprisingly he responded to me and told me something along the lines of, "Dude, fuck that, just pirate Photoshop." I never looked back.