I have a real disdain for tags for user generated content. Every site I have seen use tags I end up having to wade through a ton of spam or vaugly related content to whatever tag I am looking for. The only exception I can think of is Hashtags for twitter but that is due to the character limit.
I understand that people see tages as a solution to crossposting but I think people overlook the benefits of crossposting. Being able to have seperate conversations on AI art in an art community vs programming community is more useful to me than having a single post that both communities comment on.
I'm not saying it couldn't work but I don't think it would be as trivial as that.
I also like that subreddits were able to foster their own communities with different expectations and cultures. I feel like using tags you are more or less forced to have a single overall community vs many smaller ones.