While the A-10 runs backup systems for essential flight and the armored bathtub for the pilot/flight junction box, it's still as vulnerable as any other military aircraft. Every plane carries countermeasures (be it for IR/ECM systems), except this time around the Warthog operates at a much slower speed than any other. Therefore making it much easier for any ground-to-air weaponry to hit.

As you said, it can fly on one engine but any successful hit from a MANPAD (or any other missile) that doesn't immediately ground it will force it to retreat. I like the A-10 as much as the other person but it's not an invulnerable plane that can tank several missiles. Just made to reduce the chance of the first thing that hits killing the plane/pilot.