I plan to do a pc upgrade very soon. Alongside that I plan to start with Mint or Fedora. Is there any real or big downsides to dual booting ? Aside from the harddrive space lost ?
If I like or really like my time with Mint I would probably switch permanently but I felt like I wanted to test it for a couple of months before making a complete switch.
Im a big 
so that impacts my consideration. Dont really play much multiplayer shooters so I dont have a problem with kernel anticheat games not working.
The sad thing about dualbooting is as follows: Imagine the annoyance of dealing with one OS. Now you have two OSes, so you have double the amount of annoyance.
If it's just to get a feel for things and see if you can make the switch full time, then I reckon go for it. I wouldn't recommend it long term though.
Absolute worst case scenario you have to reinstall the OS, which isn't too bad as long as you have 2 hours to kill and a different working computer to use to flash a USB.