The problems with GIMP, the last time I checked, were - no support for 16 bit, problems with color management, slow (photoshop is extremely optimized, and now has GPU support). Plus I hate the default UI - they need to redo it completely.
Proper 16-bit support is the big thing for the next release and it's significantly less slow since it got multithreading support in 2.8. Still slower than Photoshop, though.