I can tell you from personal experience that the problem with men dyeing their gray hair and beard is that because, in general, we have shorter hair, so the roots show quickly. If you keep your beard 1/2 long, in 2 weeks, the roots will be 50% of your beard. Same thing if you keep even just the back and sides of your hair at a short length. So there's a very short window where it actually looks good. So it ends up being the hair color equivalent of a combover that doesn't really hide a balding head.
Plus, if you have any significant body hair and it shows gray as well, it makes it more obvious that you are dyeing your head.
Plus, if you have any significant body hair and it shows gray as well, it makes it more obvious that you are dyeing your head.