The E-175 has as good, if not better, low vis. landing capability to the 737. If it's ATC flow control, UA, not ATC makes the decisions on which flights get held.

I don't have the stats., perhaps the people who asserts that IAH-DCA are getting held do, but that's not my experience. Moreso, the micro-hauls to EWR/PHL and the like. The IAH flights are doing just fine and all of this may just be because some folks are jumping the ERJ's with -175's.