I don't think it makes sense to say any particular genre is harder/easier than any other. It all depends on the author's inclinations and voice.
I do think romantic comedy has some inherent challenges, because comedy tends to be distancing, and romance tends to be more all about the feels, which is up close and personal. It's hard to balance the two. If the comedy is too dominant, then you lose the emotions. If the comedy is too infrequent, just an occasional tension-breaker, then you can't really call it a comedy. (But that's true of romantic suspense too; too much romance, and you lose the feeling of suspense, and too much suspense and you lose the feeling of romance.)
Personally, I think ANY kind of comedy is extra hard in terms of finding an audience, regardless of overarching genre, because while tragedy is universal, humor isn't. If you want to make a person cry, there's a guaranteed way to do it -- kill someone the reader cares about. Yes, you need to be able to write it well, but the scenario itself is pretty much a guarantee for triggering sadness. Now, try thinking of something (NOT a pratfall, since those don't work in a verbal medium instead of a visual one) that's GUARANTEED to trigger laughter. There aren't many options.
And it gets even more complicated when you add in the element of the (generalized) differences between genders, where women tend to prefer more situational humor (something that's funny only because of the particular person saying it in particular situation), whereas men tend to prefer humor that's in the form of a joke, where it doesn't really matter who is saying it or what circumstances the speaker is in. (This is the theory, anyway, from a book called something like "I used to be Snow White, but I drifted," on women and humor.)
So, for a book that's marketed to women, and is intended to be funny, it has to be humor that comes from characterization and situation, not a straightforward pun or one-liner. What's funny in one circumstance isn't funny in another. (There are some great examples, IIRC, in the Snow White book.)
It really all depends on the author's voice and the character's sense of humor. If the author can nail both of those AND the character's sense of humor connects with a wide segment of readers, then romantic comedy can be spectacularly successful. See, e.g., Jenny Crusie's Bet Me (or Welcome to Temptation).
Plus, there's the fact that when humor doesn't work, it's worse than if there was no attempt at humor. Even a near miss is painful.
In case you're wondering, I've thought about this a lot, because people tell me my Helen Binney books (mystery, not romance, but I started out writing romance until I realized I didn't have a romantic molecule in my entire body) are funny, but the thing is, Helen Binney doesn't think her experiences and thoughts are funny. They're just who she is. If I TRIED to make her funny, I think the end result would probably be cringe-worthy.