My understanding is use of one output tube versus multiple tubes that must be "integrated" well as discussed above is the inherent advantage of a SET.
The downside is the power limitation and the distortion that comes into play if the amp is pushed hard at all.
So its a very clean pure and simple circuit design that is the benefit but the "slack" must be picked up elsewhere, by the speakers which correspondingly have to be very efficient and a very easy load otherwise you cross into high distortion territory a lot faster than with other more "scaled up" designs.
The downside is the power limitation and the distortion that comes into play if the amp is pushed hard at all.
So its a very clean pure and simple circuit design that is the benefit but the "slack" must be picked up elsewhere, by the speakers which correspondingly have to be very efficient and a very easy load otherwise you cross into high distortion territory a lot faster than with other more "scaled up" designs.