I've been using them in Race motorcycles for many years. They last a long time, and retain charge well if there's no parasitic drain. Even the very small ones we use to run 24v starting systems hold up quite well (one only takes a hit when the starter is engaged and is not part of the electrical system otherwise).
The concern I have with putting them in the aircraft is that we use special chargers to trickle charge them, in order to charge the cells equally. This is separate from the bike's generator, which keeps them topped up fine. But in certain situations where you want to charge them, it must be done with a special charger for lithium batteries.
If you let them go completely flat, they will not charge again and have to be replaced. I have had them in that condition and jump started them to get through the day, and it resulted in the battery swelling and getting hot- that experience alone makes me nervous about putting one in the airplane.
My RV8 has an odyssey battery forward and I think saving a pound or 2 isn't worth the risk to me.