Alot of these concerns are for villages or roads that are way out of Universal's purview. You give them answers for the main roads and then suddenly you get a half dozen new places and roads thrown at you. I appreciate these concerns are legit for the roads around the Park and adjacent villages, but no matter how much you reassure them that these issues are covered and Universal have a plan they just come up with new concerns.
*edit* Someone on facebook cited Alton Towers as possible comparison in regards to the traffic the villages will see. Which blew my mind, Alton Towers is a prime example of a Park with horrendous transport access, UGB is on the opposite side of the spectrum and even if the A421 flooded, it'd still be 50x better to get to than AT.
Another concern that is starting to really bug me is this idea that parkgoers will skip the A421 and just go through the villages to get access. I don't know about any of you, but everytime I've been on a trip we've never taken routes through villages when there was a direct route via motorways - the motorways are always faster and far less stressful than having to navigate some villages in an attempt to beat the traffic.
The only people I see taking the villages are the locals, or those around the area who feel they know better. Some family rocking up at Luton Airport and using a rental car will take the motorways and direct route.