My only issue with coinbase is the speed of transactions or the time it takes for transactions to get confirmed (bitcoin/litecoin). Other than that, it's all good.
The only way I can see Coinbase be somewhat responsible in this case, is when they don't include appropriate fees according to how congested the network at a specific period of time is. Another factor (when Coinbase isn't responsible) is that miners just blatantly ignore transactions, certain pools have the frequent habit of doing so. It's an aspect that doesn't get much people to complain about honestly. The far majority of the complaints all are verification related, one way or another, which is something a lot services allowing people to buy and sell crypto suffer from due to the long list of regulations they have to abide by.