The main reason for the ire over the M9 is that it is the outcome of a political decision made as soon as one M. Cullen became Minister for Transport - there is no mystery or even an element of conspiracy theorising about that, it's a matter of historical record. So, purely by dint of these political shenanigans, a piece of infrastructure got built that is, on the surface at least, far in excess of what is required, while other roads that are just as dangerous as the N9, and carry more traffic, will go for at least another 10 years before replacement.
Though I hate to admit it, Mysterious has a point - to a large extent the M9 unnecessarily duplicates other routes, and effectively undermines the logic behind much of the 1998 RNA, and the decisions that were taken even before that (mainly as to the route of the M7/M8 - if Waterford was to get a direct M-way connection, it would have been easier and cheaper to bring the M8 to Kilkenny, and diverge just south of there for Cork and Waterford). So it's an expensive way of deviating from policy, and is largely responsible for the over provision of motorway.
So while it's a sunk asset now, and it will benefit the people of Waterford and the SE hugely over the coming years, that shouldn't mean that people can't discuss the effect political clientelism has on policy decisions that cost the exchequer very large amounts of money, be that the WDC or the M9. And just because those decisions might be criticised doesn't mean that anyone is criticising the locations themselves, or the people that happen to live there.