This is why I say I disagree with the sensors, at least on how you put it here. I feel that letting the entire team know exactly where a intruder would be via a dot on screen would be way overpowered. I feel that if the sensor beeped like the ion/nuclear beacon, but louder instead would be balanced better.
I have not seen(At least in my time playing) a single SBH take down a building at any health. Whenever I try a solo attack as a SBH, I usual get gunned down the instant I set foot into the building. I usually see these rushes done with 3-6 SBH at once. Plus if I was to see a SBH walk into one of my buildings, I would be doing everything I could to stop him.(Shotguners work very well in buildings)
I should elaborate on this. When I say it shouldn't be able to protect a building by its lonesome, I mean that it shouldn't be able to defend against entire assaults like the watchtower. I feel that even if the turret was destoryable by a single person, other players would rush to the defense of the building due to them hearing the gunfire.
If your team is even trying to win, its already impossible to attack a building by your lonesome and expect to succeed, even with a nuke.
Whenever I defend I usually grab a repair tool and use that after I fought off the intruders, unless the timer is quite low on the C4/ion/nuke, then I switch to a engi.