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.
Large turbines running at close to rated power actually have pretty good efficiency. Turbines are bad at scaling down to smaller sizes due to e.g. boundary layer friction, and also part load efficiency is poor. Recuperators help somewhat, though still not nearly as good as a piston engine.
Mazda addressed both of those issues, Rotax addressed both of those issues, Norton addressed both of those issues, NSU addressed both of those issues, Comotor addressed both of those issues...
The issues remain fundamental, and largely unaddressed.
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.