Meat is probably the wrong term, because to be a complete diet, the entire animal needs to be consumed - not just the muscle.

As with all things there is a spectrum of adherence. Some carnivores just like eating meat in a otherwise omnivore diet, some are strictly animal based products only.

There are some differences in vitamin availability (C comes to mind, which is actually in liver, but not to the currently recommended levels) - But curiously carnivore only populations like the historic Inuit did not suffer from scurvy. This is a area of intense debate, where we could discuss bioavailability of different food sources (liver is the supremely best bioavailable source of nutrition I’m aware of)

I think the most popular use of pure carnivore is for people trying to fix gut / bowel issues - carnivore can be a diagnostic tool - eliminate all foods except one, once things have stabilized, introduce foods back in slowly until symptoms appear.