In the case of Revell's Nomad, it is the fit and proportions of the kits. If you watched the video, all the issues with this kit also apply to the other Revell Tri Five kits from that era.
And for the Monogram, it is detail and body proportions that make it a negative kit for those striving to build something more accurate.
The best way to know is by building them yourself and then making the call, and most of us who are calling them out, have built them in our past. Some of us multiple times.