There is a big difference between union and corporate influence. Union influence is based on the ability to motivate their members. Corporate influence is based on their money. This ruling will have a much smaller affect on union behavior than it will on corporate.
I don’t think a focus on “personhood” is really salient to the Citizens United decision, though. The Court did not find that corporations were “persons.” It referred to them as “associations” and stated that the First Amendment extends to such associations.
This is not to say that there are no grounds for criticizing the decision as a matter of law. I just don’t think this one is the right avenue of attack.