I don't think paleo has been popular enough for long enough to generate a significant community of people who experienced a failure of the diet. So give it some time ;)
Also, it's a lot harder to "fail" with paleo as spectacularly as you can with a poorly constructed vegan diet, since paleo is by definition far more diverse than any vegan ideology. You can certainly fail to see benefits, especially if you start out healthy (and so see no gain) or have real medical problems (and so diet wasn't going to help anyway), but I don't think I've seen many account of people who took a stab at paleo and then became seriously malnourished the way many ex-vegans tend to talk about.
To unfairly use Grok as an example: he talks about eating pounds and pounds of fruit. If you don't eat enough, or the fruit isn't ripe enough, you will fail as a low fat raw vegan with all the health disasters we hear about. I still count that as a diet failure, because the person tried and failed to comply. Grok or a LFRV would claim that the person didn't eat enough (which would be true), and so doesn't count as a fair test. I disagree.
I am counting anyone who fails while trying what they believe to be the paleo diet to be as a failure. Everyone here has probably told someone else, "Wait, that's not paleo" when they tell you what they think paleo is, but if we're to evaluate fairly the rate of failure, we have to count those people as well as anyone who was perfectly orthodox and failed. There are plenty of failed vegans out there who probably did the vegan diet wrong, but who still count as vegan-fails. Consistency is key.
So my definition of a failure has two requirements:
- The person made a real effort to attempt a diet methodology which reasonably fit the definition of the diet (ie paleo or LFRV or whatever)
- The person perceived that the diet was net detrimental
With that out of the way, the failures I've read about tend to fall into two categories:
- Paleo didn't cure [insert health problem here]
- I'm not
as energetic/athletically capable as
I was before
1 may simply be unrealistic expectations, and presumably is a criticism of any diet. So I'm not sure that's really a failure.
2 is more along the lines of what you're looking for, I think, and it's probably a valid criticism of the basic paleo principle. You certainly CAN perform athletically on paleo, and it need not limit you at all, but if your take on paleo leads you to low-carb or some other restriction that prevents you from performing, you're not going to be a world-class cyclist anymore (or whatever).
I've yet to see an account of someone who achieved severe malnutrition on a paleo diet.