I had bad experience with expensive tools in the past, where OS names were listed in the white papers and sales publications, even in the header of scripts, which simply did not work or were a pain to get to work.
Some monitoring solutions can't come out of the box as some demands for applications etc. is far too special so you often have a lot of coding or at least configuration works.
Some companies even charge insane prices for additional probes/modules/plugins/spys (whatever they call them), that are so badly programmed or simple, you could think they are making a bad joke.
I would always setup a detailed Prove of Concept, invite the company and have detailed things tested, before buying anything. The sales often promise a lot, while the techs take the pain or the hotline/support is pushed to the front to block off the customer more or less.
Nagios, as a free tool for example, offers a lot of plugins that cover most things, but the plugins you can get for free are from very good to flawed. Again, sometimes you have to write stuff on your own but can offer them for exchange, if allowed
my 2 cents