The plants wouldn't help the NCR more than they would any other faction. Besides, rads are not a real concern in NCR terrotory from what we've seen.
The NCR has pack animals and meat. In fact, Brahmin are a large part of the economy.
The NCR has more manpower than any other faction. Gen 2s are no more advanced than most prewar robots. There's no need for them in the NCR economy.
Institute tech is based on the whims of mad scientists, a small isolated group in extreme circumstances. It isn't catered to the needs of an actual civilisation. More importantly, were the NCR to have a large enough presence in Boston to incorporate local groups, they'd already have solutions to all their major problems. The NCR is on a modern technology level, and an NCR controling both coasts would essentially be a United States.