No no it did not. Well perhaps careful thought but there are so many flaws throughout that book which suggests a total lack of research.
Like how much damage the human body can sustain and still be considered a threat. For example if you got shot in the arm and couldn't feel pain? You would be lucky to be able to move your arm correctly as the muscles would be torn up, worse by even trying the muscles get torn up more. If you could move your arm you wouldn't be very strong with it. It's not a matter of pain it's a matter of your muscle has a gaping hole in it.
Exactly this post. Go read it.
Alright, Solarium basically makes them immune to rotting as wildlife naturally avoids them and they're poisonous to everything. Except humans. We zombify instead.
However it does not address the degradation that the human body naturally goes through every time you do well anything. As well as all the minor injuries that you get when you can't feel pain and lack the intelligence that slugs have.
So yeah they might take a while to rot but they will cripple themselves very fast just by not resting. Plus they are very easy to just walk away from. Hordes of zombies don't happen because they are hunting prey that are faster then them, smarter then them, possess tools and the ability to create barriers, and possess weapons that can easily destroy zombies.
The problem with Brooks is that he went out to make his zombies realistic and still an actual threat. Naturally he failed as I think that's pretty much impossible. But he didn't realize that he failed and that's the problem.