What game have you been playing?
The Brotherhood of Steele don't get in the habit of attacking anyone with advanced technology, the last thing they want to do is harm anyone with high intelligence or reduce the chances of humanity's survival further by causing population attrition.
If your sole experience with the Brotherhood of Steele is by their former leader Father Elijah, then that might have given you a false impression of their attitude towards technology.
In point of fact, he was removed from leadership because he was too radical and aggressive for their tastes.
The Enclave however, are exactly that hostile towards anyone with advanced technology, including the Brotherhood.
And for me personally, as soon as they entered the first room, they'd see everyone in BoS uniforms and one suit of power armor.
So technically, they'd be fighting with other members of their organisation and I can't see them doing that.
Super Mutants don't do so much raiding these days, the 1st Gen took it upon themselves to police the more aggressive members of the 2nd Gen (not all of the 2nd Gen were hostile).
Suffice it to say, there are very few violent Supermutants left......
I don't like the idea of being force conscripted into the Legion, so I'd have to say no to that idea.
They were the one enemy faction I actively hate and made it a point of waging a war against them in Fallout New Vegas.
On the other hand, I wouldn't mind being given an optional choice to decide which faction you ally your Vault with.
NCR would be my first choice.