Regarding the whole "zombies shouldn't be able to overwhelm armies" thought, I'd argue that it's plausible as described in the book due to two things:
1) The exponential growth function that governs the spread of any zombies means that after a certain point, their numbers would swell to the point that it would challenge the ability of a military to confront them directly. I know in the US, the government is always worried about causing a panic, always wanting to show that it's "in control", and willing to try and sweep festering issues under the rug so that the ones in power won't be ousted in the next election. If covert containment methods were showing signs of success, and you were a delegate to Congress, would you want to stake your political career on mobilizing the country for war against an enemy that shouldn't exist? As such, I can see the initially slow spread not being seen as "a threat", until it reaches a critical point.
2) Even if an army was able to confront them directly, the logistical backbone that feeds, re-arms, maintains, and takes care of that army is extremely susceptible to attacks. Even if they were protected, the consumables they transport and utilize are produced in large part by civilians... who were likely either delicious meatsnacks for the G's or have been turned into one of them. A sustained operation against an omnipresent enemy requires time, energy, and experience, all of which would be lacking.
My zombie fanatic friend has boycotted the film based on the trailers. Not read the book myself, but by his account, they removed a lot of the basic details of how the zombies worked in favour of silly overdone nonsense.
Eh, it wasn't that bad. I'm a pretty hardcore fan of the book, as well as the survival guide, and had the same concerns going in, but they addressed it alright.
