I've been eating 100% raw carnivore/zero carb for years now, and I haven't had any problems in a long time. To be honest, I'm not even sure when's the last time I ate anything that's not meat (that includes other animal products such as dairy and eggs). Obviously by "meat" I generally mean various organs, muscles, fats etc... As much of the animal as is possible. I'd say striking a good balance between fat and protein, not overeating or undereating, experimenting with different animal species, always trying to get good quality meat if possible (grass-fed, wild-game, organic, raising your animals potentially etc...), eating different organs, muscles and fats, and just in general experimenting to see what works best for you.
Both fat and protein are probably the most important things, getting a good amount of both is very important. Not having enough of either one is going to cause problems, while overeating (which is more difficult to do with protein in my opinion) can cause nausea or diarrhea in the case of fat. I'd say organs, while not as important as those, are still a very good addition as they contain much more nutrients than muscle ever does. Liver is probably the best one overall nutrient wise, but every organ is good for something. Brain is good for Omega 3's, the thyroid gland is good for iodine, blood/spleen is good for iron, bones/bone marrow is good for calcium etc...
As far as different animals are concerned, I don't see a problem with eating the same or similar species, although it's good to experiment from time to time. Maybe try some fish, lizards, fowl, insects... perhaps eggs or fish roe as well. Beef, lamb/mutton, sometimes wild animals such as deer etc... is what I usually eat.
Quality is also important, even more so when raw. I've noticed a sharp difference in taste between healthy and unhealthy animals, and unhealthy animals are certainly not nearly as good taste wise raw as healthy ones. That's not the only problem, of course, healthy animals don't just taste better, but they have more nutrients and their meat doesn't contain some of the junk they used to eat, to name some differences.
I don't know what you consider long-term carnivore/zero carb (as in how many months or years), but I will say that I've never had any problems with Zero Carb itself (some small adjustments are made from time to time, such as how much of this and that I eat). Perhaps the transition might cause a few problems, but I would say that in general and extreme diet change can cause that. I've also not felt the need to eat anything but meat for a long time now. I'm not fanatical about diet, and I have nothing against other foods, but as long as this is working better than everything (at least for me), I don't see a problem with it.
Perhaps the people on this forum that had problems with it did not find the right balance of fat and protein, organs and muscle, different animals etc...? Perhaps they truly do better with plants, I cannot say. I am not sure if I agree with Tyler that people with overly large livers will do better on a carnivorous diet, although it is possible. My theory is that given enough time, the body would adapt to be better able process larger amounts of protein and fat, regardless of genetics/epigenetics (except perhaps in some extreme cases). I also agree with Van and as I said previously, fat is very important. Eating pure or almost pure fat such as bone marrow is a great way of getting a lot fat. And as Van also said, look up some sites where there are more carnivores if you want more detailed information from different people. Most are not raw, although I did see there is a rawzerocarb subreddit on reddit now, as the normal zerocarb one banned talking about raw meat. Perhaps check out a few of the carnivore youtube channels as well.
Having said that, I think that everyone needs to experiment and see what works best for them. While I do think people should try out different diets for at least a few months, if not a year at minimum (barring any serious problems, of course), you should always do what's best for you. If you do better with some plants or mostly plants then that is what you should be eating. Although, unfortunately, it is impossible to really try everything and know with 100% certainty what is best.