Online wallets are never safe especially when they're on exchanges. The best is hardware wallets or paper wallets but they're also not 100% secure. Better is to use an old Android device, configure it once, get all the back ups and use it as a cold storage.
I never use online wallets since they're not even 1% safe, one exploit, and all of your funds can be gone. I use trezor, plus metal phrase keeper plus a swiss locker for the metal phrase keeper.
Does the metal phrase keeper worths it's price? If you engrave/scratch it into a piece of metal label (like dog-tag for the millitary troops), it also can hold your seed safe, and also fireproof (waterproof, shockproof, etc
) You also won't need the swiss locker if you change the order at least two words in the seed, just remember the original order...