I use Guardian shield plenty when I play support so I disagree that it's bad. We haven't really had an offense based shield yet which could be interesting.
I don't think it being a bow is a problem, there are plenty of ways you can make a bow fit well with engineer. They just really badly implemented the one they gave.
My only criticism is the AA sounds too similar conceptually to Engi pistol which also uses fragmentation rounds. Maybe change it to something like "hollow point"?