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.
Use two separate drives to save yourself the headache of working with partitions. The only “headache” I experience is the pc clock will be wrong when I switch OS´s
and if you connect a bluetooth devicw to lets say the windows installation, you need to completly re-sync the device when you switch OS. every sjngle time
The clock thing is because windows sets the system clock (RTC) to local time and Linux typically sets it to UTC. Both can be tweaked to keep time the other way though.
this is also fixable. at least with ubuntu and windows 10 I was able to configure my system in a way that this was permanently solved.