i race a kyosho lazer with no undertray and i dont know how but i got a huge dent in one of my 6 cell packs. im sure if this was a soft lipo it would have melted half my car. yes you run an undertray but it can happen.
There is a difference, your NIMH hang right at the lowest level of the chassis, Mine are about 1/8" above the lower side of the chassis. I do intend to get the 1/16 piece of lexan to underlay the Lipo, but just saying i don't see a problem, unless the chassis breaks