NVH would go through the roof with a solid mount or even a poly bushing. Firming things up is fine but not everything needs to be hard mounted.
So that's a common fallacy - it happens when people mix up which bushings are in question. Specifically rear subframe bushings vs. bushings for the powertrain/drivetrain.
Many OEMs actually use different (and solid) bushings for performance variants. One that would come to mind is your regular BMW 3 Series vs M3 on the F8X and G8X platforms. Another example would be the rear subframe bushings on the 6th Gen Camaro.
I've actually replaced my factory rubber rear subframe bushings with solid rear subframe bushings and it results in almost no NVH increase. Now if I were to replace the following bushings, you would most definitely feel an increase in NVH:
1. rear differential bushings
2. rear control arm bushings with control arms using uniball bushings
3. solid engine mounts
Suffice to say if people are not experiencing any increase in NVH from these rear subframe bushing inserts, chances are NVH is fairly
inelastic to the hardness of rear subframe bushings.