I heard that investing in bitcoin is dangerous. How true is this?

That really depend on where you invest your bitcoin. There are a lot of HYIP out there and there is a really big risk of losing your bitcoin in investing on HYIP sites so if you want to be safe from them, then you should avoid investing on it. There are a lot of danger on losing your bitcoin like losing your private key or sending it to someone you didn't know etc. So invest what you can afford to lose.