Boston has a huge manufacturing base, and a lot of the factories are in halfway decent shape.
If you have Danse tag along a lot he mentioned at numerous factories how they can get it running again to produce new weapons. Some factories he mentions could be used for power armor production.
Most importantly is the take over of Mass Fusion as it is actually still a working factory at its core. Sure the accounting area is trashed, but there is so much potential.
I can honestly see the BoS gaining so much from the Commonwealth.