One aspect is that whatever a solo performer does to add variety to his work, it still comes from him, and that is a basic limitation.
The duo, by comparison, always has a dialogue at the very least, which has a choice of relationships: the interpersonal interaction of superiority/inferiority in the music, enharmonic concord or discord, and the cross-relationship of every dynamic each makes as a solo artist. Another aspect is that the success of the solo performer is the compliment of his cockups, whereas a duo can cover for each other, so it's the compliment of the product of his cockups. If, for example, a singer makes a pig's ear of one song in ten, then the solo is 90% OK, whereas the duo in 99% OK, in other words the odd problem gets forgotten about whereas the incessant glitches of the soloist makes them vulnerable.