Aside from niceness, NDAs and fear of litigation, there are other 
factors that influence the lack of detailed product information and 
critiques.

A lot of patrons may use library systems but often their interaction is 
limited and indirect with a specific vendor's product.  It is often 
rebranded, customized and integrated with other products to meet a 
specific library's needs.  The patron often has no idea which product 
they're using and might make only occasional use.  Given the situation, 
I wouldn't expect the volume of popular blog posts, comparisons and bug 
reports as for something like Firefox or Microsoft Word.

Even comparing to other back end software, I'd expect something like 
relational database or payroll software to see broader use and adoption 
across industries than certain library systems. With more use and 
evaluation, I'd expect to see more public feedback and complaints about 
the software in a Google search.

The library community can be relatively small, specialized and niche 
compared to other markets.

As a comparison, I once worked for a large original equipment 
manufacturer (OEM) that sold computers to end users.  They were looking 
to move from their home-brew phone technical support, ticketing and CRM 
system to a commercial product, preferably with focus and experience 
with our industry needs. I was involved in some of the evaluation and 
meetings with vendors.

We ran into some similar problems of not being able to find many public 
critiques or much information about significant bugs or problems 
(NDAs?).  Of course, vendors had their lists touting prominent 
customers. They even gave us a contact or two at companies using their 
products who would say generally nice things about them.  However, 
really useful information was most likely to come out of our own testing 
and evaluation, along with informal back channel sources, not from the 
vendor or public information available on the Internet.

In other words, I think there is more at play here than librarians 
simply (and stereotypically) wanting to play nice.  It seems to be more 
or less the case with other niche products in other industries, too.