I have the same problem unclipping while riding up very steep hills on my mountain bike. (I don't ride many steep hills on the roadie.) For those who haven't experienced this, here are some of the issues:
  • Pushing so hard to maintain enough momentum to avoid tipping over that we don't have even a micro-second to slow the pedals enough to unclip
  • It's tough to reestablish contact with the pedal in the micro-second after unclipping to maintain enough momentum to slow & stop the bike.
  • Sometimes my legs are so tired that I've lost the coordination to unclip on the first attempt. Even one unsuccessful attempt is enough to slow the momentum enough to make me tip over.
  • Riding a mountain bike uphill on single track doesn't allow for steering across the trail to unclip & stop.

This difficulty with unclipping on hills has prevented me from tackling a a very long, steep hill on my favorite trail. I've decided to attempt part of this hill on my next ride by lowering my saddle a bit & riding unclipped with the pedals centered under the arch of my foot / shoes. My plan is to continue riding parts of this hill until I develop enough strength to ride the whole thing. Once I'm stronger, I'll try riding it clipped in. This process could take me a year or more considering even my very strong husband sometimes can't make it all the way up this hill.