Sparks won't be an issue unless fuel is pouring out of the wings.
Impact loads will be much less on a flat tarmac runway. Grass strips are not flat. With the belly of the aircraft sliding along the surface, the bumps of a grass field at 70kts are far more likely to break up the aircraft and the occupants.
I'll accept that there's less breaking on tarmac (than dry grass), but then tarmc runways tend to be quite long.
Given the choice of a gear up landing at Headcorn or Biggin, I know which I would choose.