Its very easy to use and my goto image editor, but I say that from a position of familiarity of having learned where everything is and what all the keybindings are over many years.
In contrast, Krita seems like a far better image editor, but because the interface is bewildering to me, I’ve shied away from it.
I was also wondering about this. Flatpaks apparently come with more libraries to interact with other Flatpaks, whereas AppImages tend be purely app-specific and their libraries are compressed for their usage only.