Its OK as far as online wallets I guess and I use them a lot with GDAX to buy coins
BUT REMEMBER
If you don't have the keys, you don't own the coins. Always remember that.
No I wouldn't say its ab OK wallet, an OK online wallet would be maybe blockchain.info which atleast gives access to your private keys. Coinbase should not be used as an wallet at all IMO, just used as an typical exchange, buy/sell bitcoin and transfer them asap to your 'wallet'. I would say any online wallet can't be considered safe, best using an offline desktop wallet.