Dangerous in what way?
Gas fumes or exhaust can build up real easy...it's the lowest point in the shop.
If something happens, it's not always easy to get out.
Would not seem a problem if the car was not started or ran when in the pit. As for other fumes or vapors one or two small fans would assure safe oxygen levels safe at all times if ran while occupying the space. I would love to have a pit.
Yeah, don't tell your insurance company, lots of accidents as result of pit. NEC says its a class 1 Division 1 area in a repair facility, Explosion proof lights, all piped in galvanized pipe and seals offs. Lots of cars driven in to pits, lots of people have fallen into pits.
But I can see your point, pits make working under a car better. For what building a pit would cost you could buy a good used lift, maybe even a new one?