It just ends up being incredibly cringe IMO. A universe where all the same people are born (either the same couples must end up having sex on the same days, or all timelines in which a genetically different child is conceived are cosmically censored) and all the same ships are built, but everything is evil exactly all of the time. There's an Evil McDonalds and an Evil Starbucks there. That's why for me the Enterprise take was one of the better ones: a self-indulgent whackadoo sidetrack that just let the cast, who weren't even going to get to take the lead roles in their own finale, dork around being cartoon villains. And it had the evil intro, which is just cheesy fun whenever it's done.
Discovery used the Mirror Universe for comic book retcon silliness, and in a better show that wasn't somehow simultaneously trying to be one of the serious ones, that might have been okay too, but this was not that show. Evil Georgiou is at least a fun character to have around, and Tilly pretending to be evil Tilly was exactly perfect. The Mirror Universe is also the device used to remove any nuance or threat from Lorca shortly before removing him from the show, though, so even as a device it causes at least as much damage as it does good.
In general, I think it's one of the dumbest recurring devices in all of Star Trek. If you want to do the alternate wacky world that comically parallels our own sort of thing, just do an episode as a spaghetti western or a 90s sitcom. Like, probably not on the set budget of Enterprise, but Discovery put an awful lot of effort into their Mirror episodes.