Since the beginning, Coinbase has been kind of fishy... Every time Bitcoin would drop sharply in a short period of time, it would shut down all transactions (not sure if it happened today, I haven't been following closely)... In 2012 or 2013, they canceled an order I placed because the price of Bitcoins was too low.

I use GDAX to buy coins but I transfer them out right away.

Now they stopped BCH/BTC trading and it appears that they also freeze the prices to control drops (https://www.reddit.com/r/CryptoCurrency/comments/7lch2d/coin...).