While I generally agree, that is my answer. There are not "genres" that RPGs can't handle but there are cross genre conventions that RPGs cannot handle in the general case of being a social game. Genres are so broad that I don't feel like they are a useful description of the limitations of an RPG. You can handle for example 'Science Fiction' within an RPG but I can think of a lot of Sci Fi novel experiences that you can't deliver as an experience of play in a traditional RPG. You can deliver 'Starship Troopers' or 'Star Wars' or you can deliver 'Not Dune but within the Dune setting' and possibly even 'Not Shadrach in the Furnace but within the Shadrach in the Furnace setting', but you can't deliver 'Dying Inside', 'The Disposed', or 'Time of Changes' in any meaningful way. For lack of a better word, let's call those things much more narrow than a genre "themes".
I will put a caveat in that that I think an RPG can handle just about anything for the case of 0-1 players (or 1-2 participants in the case of GM-less games) and that many of the critics of traditional RPG play (Ron Edwards for example) are really thinking about games with at most 1 player and wondering why games with 2 or more players can't produce the same results or handle the same content. For example, I suggested that games like Wraith the Oblivion or Monsters and other Childish Things are unplayable as written, but really what I mean by that is unplayable for a group according to the micro-fiction. You could totally run those games and deliver the experience described in the micro-fiction and examples of play if you had only a single player being catered to by a game master. Or you could deliver some sort of functional game that did not correspond to the fiction described by the rules but if it isn't the fiction described by the rules then either there is a disconnect between the intention and the implementation or that fiction isn't an RPG fiction.