Hmm well I think it depends on the person. For example, there's no way I'd be able to run a live group, because I don't ever have significant blocks of free time to set aside. So for me, you could say that face-to-face GMing is harder (because it's impossible).
There are some unique challenges to PbP though, such as:
- The slower pace can make it harder to keep people's attention.
- You need at least passable writing abilities.
- Organising and managing information in a purely digital space isn't always easy (though this one cuts both ways).
- Some people might find it harder to communicate emotions without being able to use body language. This can also exacerbate OOC drama if people misunderstand each other.
- Things that slow the game down, like initiative order or things that interrupt other people's turns become problematic in ways they wouldn't be at a table - which means the GM has to take them into account when planning.
- You'll usually be gaming with strangers, and there's generally less investment in the game.
- You can't play when the site goes down.
It's nothing that can't be overcome, of course. And there are positives as well.