Certainly road games are any less than predictable wins for the visiting team. Most major upsets occur on the road (in the NBA). In football I'd assume that is likely as well, but there is a lot of difficulty in determining a team's true skill.
but perhaps there are instances where a good team hosting a really bad team on the end of a road trip has virtually no chance to win. This would require some research but is very interesting.
Conversely, perhaps really bad teams have a better chance of upsetting or covering the spread against a *really* good team than just a good or above average team.