no cells do not warm up, they are kept warm all the time using insulation ( consider that on the other temp range is a liquid hydrogen and it it stored for quite a while in cars ), the insulation adds cost, but less, than difference between lithium and molten salt cost. for that reason molten salt vehicles work ok at - 40 C while lithium cars just stop working.
also molten salt does not explode, though can splash, but in buses it is situated such that it is last thing to be hit from outside.
li ion batteries actually is a sort of relatively recent breakthrough, that is why so much hype around.
but there are other alternatives, btw, such as aluminium air or zink air batteries (and even li air batteries). there are still problems with them - but at the cost/weight side - they are the only batteries which can compete with gasoline, so it is really strange that instead of attempts to improve in a way which can finally resolve gasoline dependance relatively scarce lithium is wasted.