[andygates]

They're very low power, make a lot of waste low-grade heat, and use an inefficient electricity generation approach.

[masklinn]

> make a lot of waste low-grade heat

That's probably the biggest issue for deep space it's extremely difficult to shed heat in hard vacuum (without ejecting mass and its heat with it) as you can only radiate it away.

[acidburnNSA]

And radiative heat transfer scaled with temperature to the fourth power so it would be easy to do if you had materials that could handle really high temperatures for long times.

[wbl]

But then your Carnot efficiency goes way down.