Im aware that some areas may bend and other unwanted things can happen.
The main area I was struggling with was at rear trying to get the axle stand under on the pinch point.
I think last time I used some wood to lift with when I done the brake drums I do recall that I had a bit of an issue tho the first time I tried I let it drop onto the axle stand meaning to just drop the car back on the wheels and it hit the sill.
The area is flat that I'm working on with a very slight incline and other than that man hole cover it's a nice new surface only being layed a few year ago.
I would always avoid working on a soft surface.