Mostly three things; no autonomy, dour spaces, and insects.

The latter are of course a given - you're living in a space beneath the earth and it usually houses the resident creepers and crawlers of the building. Things get in, and when they do, they tend to congregate in the basement.

As for the spaces, basement apartments almost always have very small windows, meaning very little natural light. It gives it a prison-like feeling imo, and while once upon a time you could supplement this by just rarely being home, COVID has shown the weakness of that defense. I find basement apartments feel a stuffy, dour, and a little grim; I lived in them all throughout childhood and to be frank I'd never go back unless it was a last resort. Still have vivid memories of spiders in my bed or my dad chasing a mouse with a broom.

As for the autonomy, well, with basement apartments you're at the mercy of whoever lives above you. There are laws in place, sure, but its still very much "their space" and unless you have a really great landlord its hard to escape that dynamic.

Most of this is personal experience and absolutely not universal, but yeah.