Per the Magnuson-Moss Warranty Act: When accessorizing your vehicle with aftermarket parts, your warranty claim cannot be automatically denied, nor can your warranty be voided if you install non-OEM parts in your vehicle. The burden is on the dealer to prove the aftermarket parts caused the failure.
So just the act of simply installing a K&N CAI or filter will not automatically void the warranty. I've used both K&N CAI's and replacement filters in many of my vehicles for over 20 years, and never, ever had a problem that was directly related to the K&N product. Most of the time the problems that are linked with these filters (oil on the MAF) result from someone who has cleaned and re-oiled, or in this case, over-oiled the filter.
There are a myriad of differing opinions on whether CAI's and K&N filters are beneficial or harmful to one's vehicle. Do they really increase HP/performance? K&N boasts that they do, and has dyno numbers to back it up. Does that apply in the real world of everyday driving? I think that's very subjective, depending on each driver's own "butt dyno" feeling. The only way to tell would be if each person who bought one conducted their own before/after dyno test. Do the K&N products improve MPG? Well, like any mod that claims to do so, maybe or maybe not. It's hard to determine because the one thing in a vehicle that has the most impact on MPG is the weight of the driver's right foot. I don't care how many mods you add that claim to improve MPG, it all comes down to individual driving habits.
I personally like the K&N products because of the cleanable, re-oilable filters. I like being able to buy one filter for my cars and never having to buy another one again. And I've never had a problem with over-oiling/MAF contamination on any of the cars in which I've installed a K&N product. I'm very careful with the amount of oil applied to the filter, and I then let it sit for a long enough time to be absorbed by the filter itself. I also like the K&N CAI intakes for the deeper "growl" that they give to the engine. That's just my preference. I'm a car guy, and I like to hear the sound of my engines.
So in my experience and opinion, K&N products cause no harm to a vehicle when used properly. So if you want to use them, go for it. Your results may vary depending on maintenance and driving habits.