From scheib:

Unretained always makes a reader wonder, "Is it clear that this class will outlive the callback?" Well, to me it isn't 100% clear. I'm guessing it is because mojo wouldn't dispatch the callback if the class didn't still exist, but I'm not clear on when/why a reader knows that. I didn't see it in the mojo primer, it's not obvious elsewhere. We should ping mojo folk so that this pattern is either documented in mojo primer, or to have the code read in a way where we don't have this (e.g. weak pointer).