if it looks likes its a really thick peice of metal then its ok to lift the car via it.
if it looks like it would fold flat if whacked by a large hammer then id keep clear.
aim for the reinforced areas, like front/rear axle subframe or its mounts.
sills, under floor, front cross member & some of that box section under the floor(some will dent if not supported by decent sized bit of wood to spread the load) should all be avoided if possible from taking the full weight of the car on it.
i did once jack up my old rover 215 up, i wasnt paying attention and jacked it up under the floorpan, the car went up about 10" before the floor seperated from the sill and the car came back down suddenly... luckily we were intending to do some welding that day, just ended up having to weld the entire floorpan back down and to the sills...
wasnt on the schedule..