Bad time to ask, class balancing is completely off at the moment as all classes are overpowered (to reduce the shock of the HD revamp). Also there isn't really any tough solo content in this game for a long time, the only challenges come from attempting group content solo. Cappy has always been a pretty sturdy class, but was lacking damage and on-demand self-heals. Both of these weakpoints have been mitigated with HD (at least for now), so cappy should be a pretty capable solo class at end-game (haven't leveled mine back up to cap yet, so can't confirm first-hand).
Used to play: 85 Champ / Captain / Runekeeper / Guardian, Guild Master of everything but cooking.
Playing now: Hellcat / King Tiger / GW Panther / IS / KV-5 / M4 Sherman and more