Modern vehicles are protected from the factory and you don't need it, says Consumer Reports. You don't need additional undercoating, and some types can trap water and rot out your car prematurely. While it won't "void" your entire warranty, it opens the door to deny any warranty claims impacted by whatever undercoating you used.
When I asked, considering I operate in "salt country, my dealer also said that it might affect my warranty. While I appreciate the reference to Consumer Reports, they're not making the payments on a depreciating asset. I'd like to know exactly why a product like Fluid Film would have any impact on warranty coverage.