So they do have some problems, but still almost all battery backup is done with lead acid (with lithium-iron catching up as a good replacement). I think most of these li-ion backup batteries are more about using surplus production when you scale your battery production faster than vehicle sales.
They let out fire-hazard gases when they charge. You need to maintain their internal water levels. They tend to spew acid on their surfaces. Their voltage droops as you use them. They can't tolerate deep discharge at all, and in general wear out quite quickly when heavily used.
LiFePO4 is a huge improvement over lead acid.