No, but they shouldn't claim their training approach is optimum either. Some of the leading advocates of Crossfit are even claiming that the best distance runners would be faster if they switched to Crossfit.
On the subject of plateaus, this is well known in many sports and not just strength training. It's particularly true in distance running. Taking someone with a good general base you WILL see sharpening and improvement through the addition of intensity and intervals. This is part of every well designed training regime with periodization. But the continued application and increase of intensity will lead to staleness, burnout and injury if done for too long.
If people find Crossfit to be an great way for them to improve their health and they enjoy it, then that's awesome. If they claim they've found the universal secret to training for every sport, they're delusional.
Another sign that some Crossfit advocates are delusional is the number who have switched to a 'Paleo' diet as some magical secret. Again, it's no surprise that cutting out processed crap from our diets is going to be good for you, and some people do better without grains or dairy. Big duh.