Everything looks great when you don't have any issues.
But what when there are any? Then all you can count on is a professional approach of their support.
So from my own subjective PoV:
Bittrex and Blocktrades replies instantaneously.
Changelly made a full turnaround within less than 24h.
It took a bit longer for Shapeshift but still good enough.
Getting reply from Poloniex?
Well, there's a game: "Never I have ever..."