I had a similar experience a while back where it seemed to be the case that I ordered 1 BTC, but then only received ~0.7 BTC because the exchange rate had changed. More recently, however, when I've ordered 1 BTC I received 1 BTC x days later even though the exchange rate had changed (and not in coinbase's favor). So it seems like they've changed how they deal with the large latency in transactions--for the better.

Also, they're doing something called instant pay, which could help normalize expectations of what happens when you buy BTC. I wonder how that works with credit card charge back problems?