maybe productivity is irrelevant, and it's simply a case of being overwhelmed because your learning all of these at once?

productivity is never as good when your learning something vs when you have mastered it.

imagine your working on a team, and most of the fellow devs are comfortable with many technologies. now compare this to a team where everyone only has one role or technology. in this scenario ( certainly exceptions to this but ) in my experience the team where people can work on anything is leagues more productive.

I think that a large portion of this is attitude and not skill. to be full stack you often have to be ok with diving into a task that is not your specialty. not everyone is ok with that.

being a specialist is hard these days. I have hired specialists before but they have to add something very special and unique to the rest of the team that very few people can do.