I'm useless as Engineer, but I don't say they aren't good.
I'm GREAT as an operative.
I played several games as a Medic (because my team was short on Medics), and did ok, then in the next match we had a few, and no Ops, so I switched (which I was happy with, because I'm Op/Medic dual-spec with a couple of Soldier abilities on the side) and I was rated best overall and best Operative. The second-highest XP was an enemy Operative who was only about 3 points behind me. We had some epic Op feuding going on. I ran Operative for another couple of matches, and never dropped below 5000 XP.
We do awesome stuff when we're doing our jobs right - if you're struggling, try another class - you might find they're better suited to how you play - Soldiers are also pretty beastly for lone-wolf "behind the lines" action, if you're good with them.