Polyurethane Motor Mount Question

I have stock spool mounts with rubber inserts in my 73 Duster and the rubber is falling apart. So I was thinking of getting Schumacher Polyurethane Motor Mounts, has anyone used them and have any opinions? Someone told me not to get them because they make everything to rigid but what is to rigid and whats bad about being rigid? They also said not to get the polyurethane transmission mount because it would just cause vibrations. I haven't really looked to see if I could still get rubber mounts but most places I have seen that have them have polyurethane and my thinking is if they were bad they wouldn't make them but since they do, they must not be.