I'd add (and the author mentions this) that most system integrators have a bias (whether financially driven or not) towards particular software. That makes it challenging to assess "is this the best software or what they pushing me to"?
I don't see how this is that much different from buying from the vendors.
For me, I usually take a vendor's customer page and start calling people myself. I also reach out to my network to see if anyone has an opinion. And if I can find a list of companies using the software (vs. who the company says they work with) then I call/reach out to them as well.