Dude, this happens for EVERY custom ROM.. of course flashing a release candidate instead of a stable release has a bigger chance to damage your phone but that's just "in theory": chances are really really low but meh, it still can happen so Devs are forced to write this disclamer to tell people they are not responsable of damages that can happen.
About the warranty void, if you wanna be more comfortable, just download an official firmware for your phone. You can flash it with odin in case your phone gets damaged and you need to take it to your assistance (of course I'm talking about soft-damages: e.g. if your phone's motherboard gets burned you won't be able to flash anything, but in that case maybe Samsung assistance won't be able to notice that you were using a custom ROM either.. so the product will still have its warranty )
Yet, flashing is cool. Stock ROMs suck. Do not overthink about it, just flash!
If there is a thermonuclear war, that's on you