Well, I've been fixing cars for 30+ years and you are right about using them on smooth concrete - BAD idea.

My driveway is an exposed aggregate - kind of a rough pebbled finish. No slippage to speak of. My ramps are the heavy-duty 2-ton rated steel type. Probably 30 years old.

Personally I like the ramps better than jackstands or a jack. If they are used properly - wheels centered - both side to side and front to back in the flat upper part of the ramp (which is actually a dished depression about an inch deep on mine), with the parking brake set, the vehicle is more stable than on a jack or jackstands.

I've pushed a car hard enough and far enough to topple jackstands or push one off a jack a couple of times. Never have managed to push one off my set of ramps though.

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown