Yes they are. An ideal stirling engine has the Carnot level efficiency, i.e. maximum theoretical efficiency for a heat engine.

https://blog.mide.com/thermodynamic-theory-of-the-ideal-stir...

Except the ideal one doesn't exist and multistage stream turbines are more efficient in practice.

Definitely not at that scale. Small turbines have low efficiency because tip speed increases quadratically as they get smaller.

And more reliable too. The Stirling does have one huge advantage which is that it is entirely self contained.

But I suppose that having a multistage steam turbine in mars in not something will see at first

You may be interested in reading Allan Organ’s books. He claims that aspirations to Carnot efficiency are a red herring and claims (with Much Math) that the peak efficiency is closer to 50% of The Carnot cycle.

Hmm. Why not talk to Etalim: https://www.etalim.com/

They invented and are very close to commercializing a direct heat-to-electricity converter that uses acoustic waves. No moving parts. Close to 30% efficiency.

It is effectively another stirling, just with membrane instead of piston, and very high frequency of operation.

Sound == heat.

An AMTEC converter may be an another "fully self-contained" solution

No moving parts is the key advantage.

Turbines are only efficient at large size (see failed gas turbine cars), while Stirling engines are very dependent on construction details for efficiency.

Nope. It is easy to make an efficient heat engine if temperature differential is so huge that it is sufficient for a steam turbine. And efficiency of a turbine will be nowhere like Carnot efficiency. Stirling engines work on much lower temperature difference, providing efficiencies dead close to Carnot levels (which is still, on that temperature difference, quite low).