I beg to differ, I have used blockchain online wallet as well as coinbase to store Bitcoin and Ethereum but I have not faced any sort of issue till now, I believe it's upto us about how we secure of keys and password because also most of the online wallets have 2FA nowadays to avoid any sort of hacking or unauthorized usage. this is just my experience but as you suggested some user might have faced issue but if we get to know the specific issue then it will be helpful in this case.
I'm glad you haven't faced any issues thus far, but the problem with blockchain.com and Coinbase isn't about the user experience they provide, it's that they're custodial. Case in point, look at how blockchain.com's wallet works:
We’ve Got Your Back(up)
Your encrypted wallet is automatically backed up to our servers. To safely store your wallet, we add another layer of security by encrypting your wallet a second time. Securely storing your wallet on our servers ensures that you can access your wallet from any (and all) of your different devices.
They keep a copy of your wallet for you in their servers, and you have
no idea how secure it really is, and nor can you verify that they're doing what they claim to be doing -- you basically have to trust them completely. Coinbase is even worse in that it keeps your coins for you completely, and they have a history of closing accounts without prior warning. It's also worth noting that most of these services won't even help you if you lose your coins, even in cases where they're the ones who caused the loss in the first place.
That being said, non-custodial wallets are objectively better because they grant you complete control over your own coins, and you don't have to worry about the custodial service being attacked. All you're really doing by involving third parties in keeping your coins is increasing attack vectors and potentially compromising your anonymity.
