Citizens United did not say "money == speech." It said that peoples' right to make and distribute movies with core political speech does not disappear just because corporate money is used to do it. If Citizens United had gone the other way, e.g. the government could have banned the Google and Reddit protests against SOPA/PIPA.

There are a variety of anti-Citizens-United positions, but the strongest (and most problematic) one is that corporations are not people, and the Bill of Rights secures rights only to people, therefore the First Amendment is inapplicable to corporations. That would, at the very least, require a revision in important precedents like New York Times Co. v. Sullivan [1], which assume that the First Amendment is applicable to corporations' publishing activities.

[1] https://en.wikipedia.org/wiki/New_York_Times_Co._v._Sullivan

In that world, any publication making or costing money can be considered political speech and censored. The Sierra club newsletter could be considered politically motivated and prevented from publishing articles unless the donors stay within donation caps

Once you give the government a tool to restrict speech / spending, it will be used to cut both ways.

Put it this way, I wouldn't want my donations to <rebellious political candidate du jour> to land me in jail because it wasn't protected speech.

Although donations to the wrong political group can certainly get you in big trouble today (if you're willing to stretch the definition of 'political group'), so...hm.