According to my experience with blockchain.info . It is a safe online wallet if you will save your password and your 12 words provided from them for the recovery of your account while if you lose them then no one will be able to recover your wallet. In this case better is that you join any other online wallet.
For long term storage never ever use online wallet and no one know about their mood and their future.
And in the last as everyone said you will never access to your bitcoin in the time when they go down and any site often face that issue so try to keep some of your funds in any other wallet too as you may need them in any emergency