there were a few concerns raised on shapeshift.io being "spotty", and transactions actually go via poloniex (? correct me if I'm wrong)
Shapeshift will send you coins from their own wallet, but use coins on Poloniex to mimic the trade as well.
Which is actually pretty good because Poloniex is a fucking shit show and paying the little extra to avoid them... maybe worth it if the transaction is large enough to not get pwned % wise by "miner fees" (which don't actually go to the miners in my experiences.)