The winning candidate spent significantly less money in this instance.
Also I tend to agree with Citizens United. Unions and Corporations should be allowed to express their views on a candidate.
If there is someone running for office and making a BS claim about steel workers, I would want steelworkers unions to have the right to advertise and inform me that it is BS.
Also if you get into issue based stuff, it becomes extremely overzealous to prevent unions and Corporations from being able to speak up during an election. Planned parenthood, for example, has a lot of misconceptions about it, they should have the right to do something about that.
I also understand the scope for abuse with citizens united, but I don't think curtailing first amendment rights is ever a good idea.
Probably worth noting that I'm not American, my opinion of this case is shaped by my listening to SCOTUS oral arguments of the case on YouTube many years ago.