I’m actually a Photoshop and CSP users that regularly report missing features to Krita and GIMP. Even Krita dev often ask community which featuers they want from CSP. Some example that GIMP and Krita does not have that I remember:
- 3D environment (available on CSP and PS)
- flexible several document views (only on CSP)
- adjustable anchor resize (only on CSP)
Fan manga and other doujin works usually depends on the original copyright holder for enforcement.
Some are pretty open for any fanworks being commercialized as long it’s limited and case by case basis. For example, Love Live franchise allows doujin manga and other doujinshi works, but not with fanmerch. Serial Experiments Lain generally allows various stuff, but not R18 content. Some others like Yakitate Japan mangaka just happy seing his works have so many adult doujin manga, and even lining up on Comiket buying them.