Eugene Volokh blogs here and here on the question, writing in light of a Washington Post article that I think is somewhat misleading in how it frames the question, for the reasons Eugene gives.
Eugene asked on the election law listserv the reason why there might be more union activity over corporate activity so far post-CU, given that the ruling presumably freed unions as well as corporations in terms of spending in campaigns. Here is what I sent to the listserv in response to that question:

During the post-BCRA period before CU, labor unions engaged in more activity than corporations, much of it through 527s. Here’s a note from the 2008 Lowenstein, Hasen, Tokaji casebook (as being modified for inclusion in the 2010 Supplement):
7. BCRA seemed to have a larger immediate effect on corporations than unions. Of corporations giving more than $100,000 in soft money in both 2000 and 2002, the amount of spending from corporate treasury funds fell in 2004 from $113.2 million (in soft money) in 2000 to $6.1 million (given to “527” organizations). Robert G. Boatright et al., Interest Group and Advocacy Organizations After BCRA, in The Election After Reform: Money, Politics, and the Bipartisan Campaign Reform Act 112, 118 (Michael J. Malbin, ed. 2006). As for labor unions in the 2004 elections, “the flow of treasury funds to political parties was also halted, but most of the dollars appear to have been spent elsewhere, either directly or in the form of contributions to Democratic-leaning 527s.” Thomas E. Mann, Lessons for Reformers, in Financing the 2004 Election 241, 249 (David B. Magleby et al., eds. 2006).
One reason that I’ve heard for this phenomenon is that some corporate (but not) union spending before BCRA came from pressure from election officials and parties. Once BCRA passed, these corporations could use it as an excuse to stay more out of politics. If that’s right, the pressure will start amping up again for corporate spending after CU.
Union spending also probably is rising post-CU more quickly than corporate spending because of the disclosure issue. Unions don’t have customers who might be alienated by political spending the way many corporations do. This is why I expect corporate spending to amp up once it becomes clear that corporations have ways (such as through the Chamber) to hide their identities, at least on the face of political advertising. (It is also why I think the Chamber has been so vocally opposed to the DISCLOSE Act.)

Others pointed out that there are certain adverse tax consequences to unions spending directly on campaigns, and that there might have been a pro-union skew because of the limited number of races which tended to involve union interests more, including the Arkansas Senate race. We will see what the fall brings.