I don't play Unity games. Not because they're poor or because Unity is bad, it's because it seems to be a crapshoot how much it clutters (or not) the pc (registry mainly). Some even save to reg.
All applications write in the registry, especially games, that's what the registry is for.
There is some truth in what you say though and that is that unity seems to promote writing data in the registry which should be otherwise be written in normal files and new developers of course go ahead and follow the simplest way to save data.
Taking that aside, Unity is a serious engine used by AAA companies as well and it has the capabilities to write saves into files just as Unreal has the capabilities to write in the registry should the developer choose to do so.
In my game, for instance, all the save files are written in My Documents/My Games/Workers of Kartedhir, together with screenshots and settings... Although the standard seems to have changed to appdata for a while, only older games seem to write in that location but that's beside the point
The only data written in the registry by my game is done automatically and it is pretty standard stuff that should be written in the registry anyways and that includes resolution and monitor used.
The reason I hate how Unity promotes the writing of save data in Registry is that a player cannot back it up as easily or even share it with other players, the only stuff that should be written in the registry is stuff that should not be shared because it would not be true in each computer that reads it (resolution and monitor used, for instance).
As a comparison I took a look at other applications, such as RPG Maker, and they do have some extra useless stuff that should be written as a file instead, I even found a "Facebook" entry with a lot of crap in it and I have not used facebook in 5 years