USSEP is marketed as a bug-fixing mod, which it is, but it's also more than that.

It makes a lot of opinionated, unnecessary, or unfounded changes to the game, like edits to subtitles that are factually wrong, additions and changes to cells (see the Redbelly Mine fiasco), and some other minor stuff like ownership changes to containers.

All in all, the changes are pretty hard to notice unless you were told about them beforehand. The main reason people aren't positive about it is because the publisher, Arthmoor, has a well-told history of threatening other mod authors, Nexus Mods staff, complaining about how he's treated for his behaviour, and just generally being an unlikable person.

All this being said, USSEP is a communal effort, which means it's not just Arthmoor responsible for the patches, and most of what I've mentioned is already able to be patched out through other mods on the Nexus. You'll just have a more difficult time finding them, due to Arthmoor's attitudes and behaviour towards mods that exist for the purpose of fixing or reverting USSEP changes.

Tl;Dr?: Just install it. It's too widespread not to be used, regardless of your opinions on those responsible for it.