Since releases are so often and with little time between them, usually new features are not tested a lot and this can bring a lot of bugs with them. Which fortunately are quickly patched in the next release.
Krita's official documentation is incomplete in some areas, especially for new features that are constantly added. But this is compensated with it's great design and usability which makes it easier to understand how things work.
Compared to to other photo editing software, GIMP is very light weight. In addition it isn't very resource-intensive, meaning it is possible to put it on a flash drive and have it with you to load up on just about any computer.
Adjustment layers offer a non-destructive way of combining different photo manipulations. Without adjustment layers the only way to see changes is by irreversibly editing the image. They are promised in future updates.