The issue at Coinbase ( https://news.ycombinator.com/item?id=6932224 ) was them taking weeks to fulfil orders, while sitting on your money, during which time the price of bitcoin was fluctuating wildly, and then processing them at the original bitcoin price, rather than the current (lower) price. End result is the customer getter far fewer bitcoins than the current market rate and accusations that Coinbase was pocketing the difference.
One of the submissions about it: https://news.ycombinator.com/item?id=6929705 Note that it took considerable pressure from the HN community (and possibly YC) before Coinbase even refunded the guys losses. They wanted to just give him a token $50 worth of bitcoins, when the profit Coinbase had made from delaying the transaction was more like $12k.
This wasn't a one off either, there were other stories on HN ( e.g. https://news.ycombinator.com/item?id=6933360 ) and threads full of people on /r/bitcoin detailing similar experiences. Nobody could get their money out or get any reply from the company and for a while it looked like it was another mt.gox. There was a total failure of support and communication.