Thank you for youre answers! 
But did you all notice, that bitcoin.de isnt "a normal Onlinewallet", its an Onlinewallet with a lot of security mechanisms. This Onlinewallet even has got an own forum and they claim to be one of the safest Onlinewallets worldwide.
bitcoin.de is not even a wallet, it is a trading platform so it is a place where you should only go and buy/sell bitcoin not store it! But did you all notice, that bitcoin.de isnt "a normal Onlinewallet", its an Onlinewallet with a lot of security mechanisms. This Onlinewallet even has got an own forum and they claim to be one of the safest Onlinewallets worldwide.Quote
So in conclusion the safety of the Onlinewallets might be a little bit lower than I mentioned, but its not close to 0%! For example bitcoin.de never got hacked and the site is over 5 years old! The guarda wallet is relatively unknown and so I dont think, that it wont be the first target for hackers, right? To be unknown is a way to be relatively secure too.
hundreds of users lose their money every month because they store it in a web wallet and lose it to all sorts of attacks including but not limited to phishing attacks, malwares, key loggers, ...in addition to that when you are using a web wallet that doesn't even give you control over your private keys then it means the security of it is 0%.
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(maybe it would be clever to buy more hardwarewallets and to devide my bitcoin, what do you think?
it is an overkill in my opinion.Quote
and a small part in an Onlinewallet (about 8%) and other places (about 9 %). Is that "good/safe enough"?
the only usage of web wallets should be for convenience not security. for example if you wanted to keep some small amount like $50 worth of bitcoin in a web wallet so that you could access it (like to do a purchase) from anywhere without having to carry anything with you.