In terms of one's sexuality, it can be. I think the replies are being to broad.
Is coming out the hardest thing for a gay person? Every person has problems, health issues etc. But the question is asking specifically about a gay person, hence the topic is about sexuality not cancer or something
I think the hardest part about being a gay person (I'm not gay) would be being taken seriously. For some reason, I see a lot of gay people always having what they say being taken lightly, even regarding serious matters. It doesn't help when a lof of a gays talk like this "So I'm like seriously pissed off about ____", "I'm like so sick of this I can't even deal" etc. and it sounds like they don't really care about what they are saying, and serious things don't come across as serious because some of them talk as if it's an episode of the Hills.
I think it's also partly because if you're gay you're often seen as "funny" as the overall description.