It’s just that he’s technically not good, that’s all. Like he drifts with time and isn’t very precise. I think the band would sound better if he were more competent. The same could be said of Kirk’s solos; even though he’s performed some classic and memorable guitar parts, on the whole they’re pretty bland.
I think for quite some time James and Jason were the best musicians in the band. Now Trujillo is far and away the most serious and capable musician among them.
That’s not to take away from the whole package. Cleary together they do (or did) something people love. But there is some objective notion of musical skill and capability that you can judge them by, and Lars is just not that great.