I am mostly using krita for painting and gimp for general editing and composing usually so I haven't double-checked every single feature on both but I have one example: krita can open a pdf but cannot save it and doesn't even have a "print" functionality. Found out when trying to sign a contract with krita on my convertible thinkpad. Gimp makes it a breeze in comparison.

I am sure there are other features missing in both directions. It is great to have choice.