It can be, if you build to operate rather than just to build. Building is a lower-risk, slower income than the fast methods, and you can level Support without fighting.
The best solo setup is a Talon SAM on your own FOB. The missiles lock on by themselves, it needs no spotter, and it fits inside a fresh FOB's 1,250 build supply. You need the $7,500 [S1] FOB and the Large Hammer.
The evidence is thin. One creator's Season 1 solo run was still profitable. In another creator's session the SAM crew lost money while a friend playing sniper and medic earned far more. The SAM operator is also defenceless: anyone who walks up can shoot you off it.