James has become more competent as a vocalist with age. My favorite performances of his are on Master Of Puppets (his classic vocals hitting their peak), Load (after introducing his more ‘tuff guy’ style, he finally makes it work here), and Hardwired... (it’s typical James, but his control is on point and he sounds amazing). He’s never been a perfect vocalist, he’s only been James Hetfield, a person who constantly keeps evolving with literally every album and it’s quite fascinating. Most of this band is fascinating.
As to Lars, I think he was a competent drummer on the first three albums, but not always quite on point. He got better on ...AJFA, but production choices marred that. Simplifying things with the self-titled record was probably his smartest move, because the drumming on that album made it sound much heavier, and on Load he actually sounds really good as well. Its follow-up wasn’t quite that outstanding, and there’s nothing to explain about the NEXT album (good drumming, terrible drum sound). DM was solid but not outstanding, Lulu had some pretty great moments, and finally, H...TSD has probably my favorite drumming from Lars since Load, if not overall. It sounds good, he’s really getting into it, and it’s a powerhouse that drives on the rest of the album. Dunno about his live capabilities so much, but I do think he’s getting more competent with age on albums at least.
As to why the band is so big... probably having five huge albums and a really memorable name helps out.