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.
hey caj here with the most built on hope and dreams setup.
uhh the downsides? besides the months I spent figuring out how to make rEFInd ( a boot manager ) work with secure boot before it was fixed so it could use shim properly, it's fine, I just.. I was a bit.. maybe a bit dumb and had to spend several hours waiting for my partitions to resize (on my SSD) so my windows just has windows in it and my linux (endeavourOS) contains some stuff I can't put into my increasingly filled up hard drive which is almost always connected to it.
overall, I recommend getting separate drives, makes it easier and less anxiety to have to think of how much free space you'll give to each OS. I recommend rEFInd, but others are fine too I just don't know how they work. Just be careful and have like.. idk, a fully conscious mind when setting up (I didn't lol).