yes, coinbase is one of the reliable companies. if they fall, we're all sol
And why you rely of them no matter how reliable they are, they can close business anytime with your bitcoins gone ? Securing bitcoin in a third party wallet in which you don't control your private keys is against the core philosophy of bitcoin itself.
There are a lot of desktop wallets like Multibit Classic, Multibit HD, Bitcoin Core, and Electrum from which private keys never leave your PC and you are your own bank just like bitcoin philosophy is.
When we will not have more than few bitcoins at a time or if we send and receive low amounts frequently, Its better to use a web wallet like coinbase, since its has to transaction fee and also i believe coinbase will not cheat or run away for some small amount, because they have good trade volume in gdax.
Also bitcoin itself is based on trust, bitcoin has value only when someone trusts it has the value. This will be the situation for all currencies, so rather downloading some wallets into our pc and knowing some technical and analytical knowledge about mining fee, Also we have to keep our pc secured from hacks and virus or else we need to keep a back up somewhere else. So i would say web wallet is better to avoid those hassles.
