Impossible 1-on-1, maybe. Bring friends/enemies and a Goliath goes down quick, especially if you can time your shots as the Goliath is deflecting someone else's.
If it takes multiple attackers to reliably bring down one Goliath, that's clearly overpowered.
It doesn't. You can take it down fairly easily with the prowler. Takes about 3 shots. The prowler easily takes out Goliaths and Mohawks. It's a great unit.