The impression I've got from reading a bit around this is that Lars can write great drum parts (especially great Metallica drum parts), and he can them play well enough... with sufficient practice.
The trouble is, so I've read, that he doesn't like to practice and doesn't do it as much as he "ought" to. Especially given Metallica's position in the industry/culture.
So he can go into a studio and record a great drum track... but it'll take him a bunch of takes to get right. For the other band members, who've put in the practice and shown up to the studio with their parts dialled in, that's frustrating and the source of a fair amount of tension. And while everyone misses a note or two in the course of an hour+ live show, the lack of practice means Lars tends to fluff a line a bit more frequently than the other members. But also, because of how fundamental the drum track is for keeping everyone else together, when Lars misses it causes more problems for the performance than when James, Kirk or Robert do.