It's a good DLC in it's own right, as in a fight through an alien ship, but the problem is that it's incredibly far-fetched for the Fallout series. While there is retrofuturism, laser guns and everything, Fallout still keeps up some sense of realism. And as far as we know there are no little green men preying on us in space, so MZ is a bit too out of place.
Another con is that it gets tedious. On higher levels there are just too many bullet sponge enemies so you can't really enjoy the view.
Personally, I disagree on how it was a good DLC. Ignoring how it was a silly DLC trying to be serious instead of reveling in its absurdity, it just wasn't that good. It was as linear as Anchorage, except it didn't have varied enemies to fight like cloaking enemies or spider mines or huge tanks, nor was the end loot as good.
Enemies were basically "alien who acts like your generic raider with a different model and possibly more HP, melee robot, shooty robot, and reskinned feral ghoul." Worse, Zeta itself was a cramped environment for the most part, with no real opportunity for alternative tactics - there was no cover to be had in general, forcing you to basically run and gun your way through everything instead of trying alternative options.