How is clicker a completion mark cheese? R Key is great because with it you can get more completion marks in a single run that it it is possible without this item and allows you to aquire more items which can guarantee a successful run.

However, the clicker can help you get the completion marks only under certain conditions which are difficult to get.

Firstly, you need find it in a run while playing as a character that already has all completion marks, otherwise it doesn't really make sense to try to get completion marks as other characters. Secondly, you have to be lucky enough for clicker to get you the character you need considering that there are 34 characters in the game.