I volunteered for this year's Boulder Roubaix road race. It was a slop fest.
Put on by Chris Grealish and
DBC Events, the Boulder Roubaix is a road race, mainly on dirt roads north of Boulder, CO. This year, it was held on the second consecutive day of nonstop rain. Temps were in the 50's. What's the ultimate bike for a race like that? I'd guess that it would be a cyclocross frame with road bike gearing and a pair of fat (700x25-30) road tires. Maybe something
like Continental Touring Plus in a 28 width. Of course, the fastest riders would probably be on road bikes with sew ups if they had access to them.
Some riders took my advice:

Everyone tried hard, and a few abandoned:

The woman's race was lead by a pro from Team Columbia. Here's the transcript of nipplework's first ever pro interview:
me: Did you win?
pro: Yea! I had a rough start though.
me: Congratulations.
pro: I think I'm going to use this spot to change.
Sorry I didn't get a more flattering picture, Mara! Best of luck and may you add the
maglia iridata to your stars and bars.