Speaking as an immigrant who is now a US citizen, I don't think this is particularly relevant. As far as immigration support from companies goes, Big Tech already offers it, so the real beef is with the federal government - and a trade or company union is hardly the best venue to have that fight. I would first and foremost want a union that protects my interests as a worker against my employer's encroachment, and it was no different when I wasn't a citizen yet.