Risky in a way that you don't hold your funds because they don't provide your keys.

We can still web wallet but choose the ones that are reliable enough or use them with a very tiny amount for transfers. I don't use Coinbase but their reputation is well known and they have millions of users that trust them.

As figured out, it's a bug. I also experienced this long time ago with an exchanger, the balance didn't reflected to my account although it's already confirmed on the network. But as soon as I reached the support, they'd fixed it immediately.