i know i'm gonna catch some shit for this but
hey! did you know that a lot of artists don't like it when you compare their stuff to someone else's stuff - specifically when you compare their stuff to that of their peers, say another fan artist from the same fandom? i know you probably (hopefully???) mean it as a compliment (or at the very least don't mean anything bad by it), but at best it comes off as suggesting the person is "just" unoriginal, and at worst it comes off like you're accusing them off copying someone else and trying to pass it as their own. fan artists are conditioned from the very beginning to know that there's nothing worse than copying someone and trying to pass it off as your own, and one of our biggest fandom-related fears is that we accidentally do that, so being accused/implied to have done so is Really Hecking Bad.
and to be perfectly honest? stuff like gangster/mafia scenarios, pirate scenarios etc. are not super original, so it's perfectly possible for several people to come up with them separately without being aware of each other's stuff. similarly, a "the skelebros were experimented on as children" scenario is also not super original, so just because one person has a semi-popular comic or whatever on the subject does not mean that every drawing of sans-in-a-hospital-gown has something to do with that comic. and to come barging in with "um this is clearly [another person's thing] and you should credit [that person]???" at someone's art with no evidence whatsoever that it's in any way related to that other person's stuff is exceptionally fucking rude. if you're going to be accusing someone of copying another person, you'd better have some real solid fuckin proof because it's one of the worst things you can accuse a fan artist of.
also just commenting on someone's art and only talking about someone else's stuff is, at the very least, really inconsiderate? if you have nothing to say about the actual art you're commenting on, then in all honestly it would be preferable to not comment at all.
for the record: obviously this does not apply to absolutely everyone. some people don't care and aren't bothered, but you have no way of knowing who those people are, so it's honestly better to just assume that no one is okay with it. most artists are too shy or too scared of coming across as mean to say anything about it publically, but you can bet your ass they are frustrated and upset about it and talk a lot about it in private with their artist friends.
(also on an extra-personal note: kaesa's fic 'Never a Lovely So Real' is inspired by my gangster AU drawings and writings, not the other way around, and she even says that in the notes of at least the first chapter! i'm not saying this to be a dick or in an attempt to make kaesa look bad, she's great and i love her and her fic is better than anything i could ever hope to come up with and that's just a fact, but the amount of comments and messages i get about how my drawings must be based on the fic is killing me inside.)