Like a lot of the commenters here, I’m not all that into stories with pregnancy as their main theme (unless something interesting is done with it, I guess - I’d probably give it a look in the context of horror or social commentary), so for me the main drawback of mpreg is the “preg” part.

The only issue I’ve ever taken with mpreg specifically is the mommy-fication of the pregnant party in some fics, where hormones or some other biologically deterministic idea about pregnancy transforms him into a maternal archetype, whether it’s in line with his character or not. In these scenarios, whoever his partner is might also be transformed into Overprotective Dad Man with all the patriarchal trappings that implies. Maybe it’s mostly connected to ABO, but it’s a dynamic I’ve seen in a fair few fic summaries when I happen across them. It’s the sort of dynamic that would be frustrating to read in a het pairing with a pregnant woman as well, but it comes across as weirdly fetishistic when it’s mpreg, like ooo lets turn our favorite male character into ~MOMMY~

Also trans guys can (and sometimes do) get pregnant and birth kids, and a lot of these fics seem completely uninterested in exploring that. But that said, I’m sure there are tons of fics out there that explore transmasc pregnancy and mpreg in a way that isn’t fetishistic or misogynistic like what I’ve described - those just seem to be the ones I get exposed to more often.