The biological issues alone would probably be an interesting point of friction.
Like, imagine living with a vegan... who's literally
never eaten meat before. Imagine how weirded out by it they would be? not to mention a pony is constitutionally incapable of digesting meat, so even having it
close to their food could cause stomach problems. Actually the whole place would probably need to be kept immaculate, just in case. And of course there's the bathing issue. Ponies obviously do bathe, but there's wonder if they do it to the same degree as people do. If they have sensitive noses a human of roommate age would smell pretty... strong, to them.
And on that note let's not forget the issue of estrus/menstrual cycles, or simply of the difference of hormonal levels. And that horses have clear aplhas while in human terms it's a bit more vague. Still, it doesn't seem to be an issue in show.
Honestly aside from the biological stuff it probably wouldn't be so bad. A human and a pony could easily get along. Heh. I'm imagining a human and a pony playing poker together, or something.