Bitcoin has been running for years and remains the top 1 cryptocurrency until now and I have never heard of bitcoin being attacked. We all know a lot of scammers out there, so maybe it could happen, but until now no one has succeeded in attacking bitcoin because it is too difficult.
Well Andreas Antonopoulos likes to put it differently and I personally believe he is most likely right. He says that we can be sure that Bitcoin gets attacked all the time. Maybe not so much anymore these days but the reason that we believe it hasn't been attacked is that Bitcoin hasn't been successfully attacked yet. In Blockchain you either crack or it or you don't, it is a binary thing. If someone breaks into your house it might be different as you may be able to see some evidence afterwards even though the attack wasn't successful.