Well, Bytehawk hit the nail really. We've rarely used them, even on years of Formula 1 games. It takes so much fiddling to get decent results you may as well rebuild the objects by hand.

They're quite good for reducing organic characters and smooth landscapes, but pretty ropey for hard-edged mechanical objects with complicated texture layouts ("UVs").

Also (techy alert) once the track surface is reduced it no longer matches the smooth 'collision' mesh, an underlying version of the track (unseen to the player) which the game uses to calculate your ship's position. The result would be ships apparently hitting invisible walls, or sinking into the track, where the two track versions misalign.