I have used K&N filters on my motocross bikes and I have one on my carburetted '80 MG midget and one on my fuel injected cavalier.
I have never had any issues, but I always clean and oil my filter every second oil change or every oil change if it is a dry dusty summer.

When I was a service consultant at a GM dealership, we found that K&N filters specifically, worked fine. Never had an issue. But with some of the cheap knock offs, like you get on ebay etc... The filters just weren't as good of quality. The Duramax diesels would suck the filter right into the intake due to the weak construction, and there were a few vehicles that had valve seating issues because of these filters.

I know that RC engines aren't the same as a 1:1 engine, but it is the same idea.

As long as you were to clean and oil the filter after every outing, your K&N should be just as effective as a foam or paper filter.
But, it would not take very much dirt to clog up a carburettor needle on these little engines, nor does it take much to put excessive wear on a sleeve.

So really, it is a potential risk. I guess it is really just up to the owner to decide if the powergains are worth the possible risk.

Just my 2 cents.