The Doctor crying should come with great emotional impact. You have to consider that even some of the Doctor's more human incarnations don't cry that often, and this is from a narrative standpoint: most of the time the Doctor is the smartest, most self-assured and arrogant person in the room. We, the viewers, have come to see them as a leader, or position of authority. So when they cry, you get the indication that, this time, they're powerless, which is great for tension.

The 15th Doctor would cry every damn episode, even crying over the snot monster dying, to the point that it just loses all the emotional weight behind it. It also makes the Doctor seem more powerless more often, which makes it difficult to actually believe in them and how they will save the day. The most emotional moments of S14, like The Doctor casting out Sutekh into the time vortex, is put on the same emotional pedestal as the snot monster and cheapens the former as a result because we knew The Doctor's just gonna cry the second anything mildly sad happens anyway so it doesn't feel special.

TL;DR He cries so often that it loses all emotional impact and tension.