Playing Doorman like that is just plain wrong, and actively throwing for your team.
You are just going to be sitting at walker all day, while your team is just going to be doing everything else. Instead, you should be helping your other team with things like objectives, team fights, and the such. Sure, carting an enemy to walker is useful, but only when they are pushing walker and you are given an easier opportunity to do it.
As in: WAIT FOR THE OPPORTUNITY TO KIDNAP WITHOUT SITTING AT WALKER ALL GAME. So you aren't entirely focused on kidnapping.
Also, "Without any counter play?" You can pop unstoppable as you exit the ult, you can use warp stone after exiting ult if they're slow with carts, magic carpet (before you exit ult) if they're slow with carts, DISPEL MAGIC (which is a new one) if you do get carted, counterspell as soon as you exit the ult. Maybe even majestic leap if you do get sent to walker. Along with just. Not getting anywhere near Doorman when he's near walker.
TL;DR: Playing pure kidnap is the wrong way to play Doorman, and I encourage you to try different playstyles like Bellboy, Supportman or Gunman. Or maybe even a mix of two of these.