1.) scams that prey upon the innocence of investors, this one keeps happening.
2.) phishing again, preying upon innocents or gullibility to get something they can get into your wallet. innocent people are a favorite to scam since they are usually new to this blockchain thing.
3.) fake sites made to look like real sites claiming to be from 'x' exchange with the logo and a link that looks like its the site but instead of being x-company.com instead its x-company.com.supercoolscamer.net (note the last part of a url is the real end point of the link and the begining can be any arbitary custom baloney.
4.) somehow chinese keep getting into areas of the internet by hacking a lot more then others, they once had warehouses farming for world of warcraft 'gold', you never know if they are also filling warehouses with desks and hackers looking for bitcoin wallets to get into.
5.) quantum computers, this might break the back of 90% or more of the current coins out there due to how fast they could bruteforce into them, most blockchain coins are not quantum proof as I've gathered, i could be wrong but this is what I've read.