I've worked with some very talented people who could reliably MIG weld to within 3mm tolerance by hand. I wouldn't be terribly surprised if you can get it close to 1mm tolerance with an automated setup like that. I agree though that the machine would need delays to let each layer cool somewhat to prevent sputtering and trapped gasses.

Also, all those printed plastic components aren't going to last terribly long with the amount of heat and UV they're soaking up from that arc. The UV from a MIG welder is serious business, about 5-6 times more intense than standing in full sunlight on a clear day if you're close to be operating the welder itself. And at the range the carriage is from the arc, I'd say it has about six months to live before it starts crumbling.