Less than legal, but completely safe, I swear. I've used this site for a while: https://crohasit.net/transformers-fall-of-cybertron-free-game-download/
Yup, it's available on Amazon: Amazon.com: Transformers: Fall of Cybertron [Download]: Video Games. For whatever reason though, it's $49.99. If it works like War for Cybertron did when I bought it back in December, Amazon gives you a Steam key and then it'll appear in your Steam library where you can download it, etc.
If you use the UploadHaven, then I'm not sure what's up. I got it done in about 30 minutes, and it plays fine. It might be something on your end