That's not quite correct, actually. Citizens United overturned a part of a law that prohibited corporations and unions from spending their money on "electioneer

That's not quite correct, actually. Citizens United overturned a part of a law that prohibited corporations and unions from spending their money on "electioneering communications" (generally defined as communications advocating for or against a specific candidate) within 30 days of a primary or 60 days of a general election. The decision is not about PACs or limitations on their ability to raise funds, though PACs certainly benefited from the ruling.