Sinovic has been just fine. While you may take exception with some of his defensive mistakes, his responsibilities extend beyond just defense. He does very well at moving the ball down the flanks to put in crosses, which is just as big a part of his job considering KC's style of play. IMO, he does a better of job of the latter than Myers does. Myers isn't bad, either. I think the lack of a consistent attack down the left hand side perhaps causes Seth to push higher up the field, leading to more defensive liabilities. Zusi and Kamara seem to overwhelmingly prefer to attack on the right side, which provides more cover for Myers due to sheer numbers. In attacking systems where the outside backs push up to contribute to the attack, you just sorta have to accept that they may, at times, not be as good defensively as the outside backs in other systems. That's why it's imperative to have a great CB pairing, which we absolutely have.
Arsenal plays much the same way, and the left backs haven't lately been the best defensively (Clichy, Cole, Gibbs and Dos Santos love to get up the pitch), but because they act so much more like midfielders, it is acceptable.