True, but we have seen the Zoldycks are also hired to off quite powerful targets. Maybe they can afford to default once or twice against especially tricky opponents, but if they fail (run) too many times, that's it. Nobody serious is going to hire them anymore. But I suppose his brother wasn't even training him to be an excellent assassin, just to stay alive no matter the cost. It simply hasn't been shown too well in the show before this arc.