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That awesome technology. I salute our robot overlords and pledge my allegiance to SkyNet. Just in case.

There has to be a downside to weld-additive construction. They'd have to do this in a vacuum or inert gas filled chamber to avoid oxidisation between layers.

I know you can't weld aluminium like this. Aluminium Oxide has a much higher melting point than aluminium, which is the main point of failure with aluminium welding.

Flux core would remove any slight oxidation between deposits on a continuous weld, or a media blast nozzle in front of the weld zone.
I agree with you if they intend to use it for load bearing structures, but it wouldn't be difficult. Just a loose seal around the work area and positive gas flow keeping oxygen out, problem solved.
The downside I see is cost. It's expensive to 'make metal' with a mig....or any welder. Electrodes/wire aren't cheap, and then there's the electricity. Bending or milling sheets, castings, or blocks is almost always going to be cheaper. This will be useful for designs that require complex interior shapes impossible to do conventionally, but not much else, imo.

I get the impression that the 3D printing and robotics aspect is tertiary to machine vision, which seems to be what they're actually researching. The video is kind of confusing that way. Cool as it all is, nothing they actually showed or discussed seemed technologically impressive (in a relative sense).