This is not actually true, mint supports x.org hacks for those things, not natively and properly, for example, the way mixed refresh rates work is like this: lets say you have a 60fps and 120fps monitor, both will actually run at 120, but half will be culled on the 60, meaning much worse performance and battery life… this becomes exceptionally bad if they are not clean multiples, say a 144hz and 60.
fractional scaling works in a similarly hacky way, it renders at 2x and then downscales, as does mixed dpi, meaning you’re paying the full rendering cost.
they kinda work, but these are terribly hacky workarounds that are impossible to avoid due to the fundamental nature of x.org. This is not something they can fix without wayland support, which will take forever to mature into usability because their dev speed is so slow.
There are stories like this for every distro, unfortunately.