Well if you're talking about two cycle motors in our 1/5th scale trophy trucks, yeah we've lost engines running K&N filters in sandy conditions, you really have to run foam and oil or pure paper filters in that kind of environment.
In most street conditions, I don't think its an issue, I've never had any issues with them on my street bikes.
The biggest issue are the K&N oem drop ins, that it actually fits in the stock airbox correctly, there are a lot of OEM drop-ins that don't fit snug and air gets past the filter gaskets, I had that happen on my old Grand Cherokee, and 4.6 F150.
If you use a K&N in a street application in a good fitting tight seal air box, you should be fine. You're not getting the best filtration possible in terms of engine longevity, only paper does that, but the performance and reusability may be worth the trade-off for you.