I use GIMP all the time. For all its advantages and the really remarkable feat of getting something as functional as this on the road without major investments from a commercial backer, it does have some serious drawbacks, at least from my perspective:
* No adjustment layers. You have to duplicate layers, do adjustments on them and then mask them out. Adobe's approach is far more flexible.
* No native CMYK support; GIMP is RGB only. This is a problem if you're exploring digital-negative based color printing processes.
* By extension, GIMP
natively doesn't support CMYK decomposition, but there's a plugin for it that's now packaged with GIMP by default. However, this does not support custom spot colors.
* GIMP's printing interface is a collection of awkward bugs and essentially dysfunctional. This has been the case for, well, ever, and it's acknowledged widely by the GIMP community. There appear to be no efforts to improve this. So for printing, it's in virtually all cases necessary to divert to a different application.
* No PostScript support.
* No support for pdf/X1A which is a major standard in the printing industry.
* No support for scanning multiple images at the same time, e.g. from an application like Epson scan (marquee select multiple areas - works fine in PS, GIMP will acquire just one image).
* No support for scanning at 16 bit color depth; GIMP acquires 8bit/pixel only.
* GIMP's color management functionality, though present and functional, is somewhat awkward. It tends to revert to a non-managed workspace by default, and color management functionality is spread all over a couple of menus.
* Image manipulation tool menus are just not as intuitive as Adobe's, although they have improved
massively over the past few years.
* GIMP's AI-assisted healing tools for retouching etc. are at the level where Adobe was around 2012 or thereabouts. These also have come a long way, though, and currently work fairly well for basic spotting & retouching of scans.
The short of it, if my work involved image manipulation, I'd happily shell out the bucks for a PS subscription. As an amateur user, I prefer to put that money into other things like film. So it's great that we have GIMP in the first place, but it's not a true alternative to PS for everyone.