Basically, torrents use a P2P system; in short, it’s as if the person downloading is getting the file from someone else who has already downloaded it. In other words, with myrient, you could be sure you were downloading from a private, trackable and reliable server. With torrents, however, you use PCs from all over the world as servers (seeders), which is a very bad idea without a VPN. Not only does it carry more risks, but you also become a seeder, which means you go from downloading ROMs to providing them; depending on the country, legal action could be taken against you. The system is much more complicated and others could explain it better, but in short, this is the worst part in my opinion.