They're not great, at all. They have very few moving parts and they are very smooth because there are no reciprocating parts.

Other than that, they have emissions like 2-stroke engines because of the incomplete combustion and we have never been able to devise any kind of material to make the apex and side seals last anything like as long as piston rings.

Piston engines are a terrible way to power vehicles, but everything else we've tried is worse.