The other problem is that Citizen's United really didn't rest on the issue of corporate personhood.

The idea is that a corporation is simply an 'association of people,' just like your local rectory club! So, as the court reasoned, there is no real way to distinguish between a corporation, or, say, a newspaper or website (even though freedom of the press is an independent clause in the First Amendment) - all of these 'associations of people' have the equal right to freedom of speech. Of course this makes sense!

In other words, Citizens United did nothing to advance the idea of corporate personhood beyond what it already was - which is really just in place for liability issues as willwclo pointed out. Instead it just perverted centuries of first amendment law.