"Then there are others where the polys are quads but the creator went bugfuck with smoothing and ended up with 40 bazillion polys on the door.  What on earth were they thinking"

Yep. There's a 56' Ford Sunliner cabriolet on ShareCG.

Great shape, but the polygon count is so high it won't even load.

Rule of thumb: If the object file for a Poser car is over 1-2 mb, it's most likely useless.