What is the biggest risk with buying a used car that was a rental?

I'd have it checked out by a mechanic before you buy. A rental was just used by multiple people. So the brakes would be suspect. But those are easy to change out/ Rental cars also have decent maintenance records.

It could have been abused and not maintained.

Yes, have the vehicle inspected by a professional inspection company or a trusted mechanic. People who drive rental cars usually don't care about them. So they zoom around and don't care if they hit curbs, pot holes, etc. because they'll be turning it in at the end of the rental agreement so who cares if there's anything wrong with the vehicle. I would be more comfortable buying from a larger rental company who has a reputation to uphold. Little places that rent vehicles, not so much. Hope that helps! Jim