Not a physicist, but I'd say it depends on what you define as parameter optimized against. For space, Id be interested in heat energy dissipated per unit time per kimogram per cargospace for exanple, where radiators suck compared to convection coolers
Depends on what you mean by efficient. I assume the previous commenter meant it isn’t efficient in terms of cost and space to build large radiators that aren’t very effective.
Yes, efficiency here is in the sense of the mass and volume of device required to dissipate X amount of heat energy, particularly in an environment where getting a lot of mass and volume up there is fabulously expensive.