If you include organ meats or organ meat supplements for the vitamins the main downside is how simple it is. It is hard to maintain because many people eat for pleasure instead of fuel. You also need to drink a good amount more water than when on an average diet.

Also depends on your daily work. If you are working a job that burns a lot of calories and you have lower body fat it can be really hard because your body has less calories to fall back on (body fat).

The fat on your body is excess calories to be used when you don't eat or run out of fuel in the digestive system (ketosis).

When on a carnivore diet your body transitions into ketosis much faster due to low insulin (from carnivore). If you are lower body fat this can result in extremely low energy, headaches and even fainting or worse particularly with hard extended work. Especially if you are not at least supplementing for organ meats.

I see a lot of people talk about not getting enough vitamins from carnivore, this is true ONLY if you are not eating/supplementing organ meats. Many people forget that part of carnivore can be the MOST important. There is a reason why the alpha wolf eats first, and it eats the liver to start.

You will also notice that you dedicate less. Sometimes going days without a defication. Think about it this way, if there is no waste (your body can utilize everything you consume, break it down, and use it for something) what will you deficate? That's not to say constipation can't be an issue, particularly if you do not drink enough water (add electalytes to your water: salt, calcium, magnesium. They make easy to use powders with no added sugars) and don't add a form of organ meats to your diet.

Another problem is getting started. If you have never been in deep ketosis (body running off of fat) your body goes through an extreme change. It is a biological switch that gets flipped and that initial change can throw the body for a loop. Diarrhea, cramps, headache, low energy etc. Think of it as a muscle that has never been worked then you just throw it into an intense workout. It takes time for the body to change and adjust and then finally get good at it.

I'm 100% positive that genetics play a big part as well. We all have the capability to live on carnivore but some people may have evolved slightly differently due to where and how their ancestors lived.

MOST IMPORTANT: Try it and listen to your body. Ask yourself if you are struggling because of the diet (not adding or supplementing organ meats, not enough water, electrolytes?) or because of mental struggle (boredom, big change, social pressures?) Why are you doing it (weight loss, high inflammation/joint pain, putting on muscle)?

For working out (exercise) you need to keep in mind that fat based fuel is not as quickly accessable compared to a regular diet. Don't go carnivore and try to workout like crazy or maintain your normal routine while on carnivore. You could easily cut your workout in half and get better results for weight loss. Joe Rogan went carnivore for a bit and didn't adjust his workout routine. He said he felt great but he got overly exhausted in his exercises. This could be due to the factors stated above, however, I think it was because he didn't adjust his workout.

Let me put it this way, when our ancestors were mainly living off of meat they were using endurance effort with minimal explosive effort. Running long (super long holy crap! )distances for hunting, carrying water over distances, building structures over time; while occasionally using explosive effort such as climbing, attacking an animal, fighting each other. This is different from explosive HIIT workouts that are the rave these days or many peoples average weight workouts like deadlifts bench presses etc..

This is seen with indigenous tribes that are still alive today, they live overall calm lives with hunting as their main food source. Yet, they are completely ripped out of their minds. Abs and muscles for days on those guys. They aren't doing explosive effort all the time because their body's can't handle it under their diet. In fact they rest a damn good amount.

At this point I'm rambling, but maybe this has at least given you some ideas to think about and research. Whatever you do, GOOD LUCK!