Unlimited spending does favor unions and advocacy groups as well. They are organizations capable of raising large amounts of money. Therefore, unlimited spending favors them because they have some what specific personal agendas that lend themselves to politics. They can pump all the money that hey have raised into politicians that promote their agendas.
Yes, unlimited spending favors unions and advocacy groups, but this is not always a bad thing. Limiting spending would allow less-powerful groups to have disproportionate influence over public affairs. First Amendment considerations allows these groups to spend as much as they want to in order to make their voices heard. Although some voices are louder than others, there should be no limit to public discourse.