The worst thing that I often hear is bitcoin as a fraud and prone to theft, but after I learned it was not true, because bitcoin is something real in an investment and can indeed give us big profits, for security issues I think it depends on each individual as a bitcoin owner where they have to be able to maintain the security of bitcoin they have from theft by storing a password that is well owned in a safe place, so we avoid theft.
It is difficult to say about the worst things that were heard about BTC. I guess it was the rumors that Bitcoin's going to die very soon. That time many people believed it and rushed to sell their crypto.