Broadly, I would agree with what has been written there. I've seen the upper mounts fail too - both using adjustables and non-adjustables such as Bilsteins. As has been written, the Hydragas units have internal damping, and in fact were originally designed to not require an external damper at all, but clearly Rover engineers wanted a little more rebound countrol. The standard dampers have next to no compression resistance - so in fact, the mounts don't have to be particularly strong!
I've not seen lower mount failure - but that's not to say that this doesn't happen.
There are of course a good number of cars (mine included) that have never had any problems (I've used Bilsteins, done over 60k miles over many years and never had problems). Plus reinforced upper mounts are available if you need to reinforce the upper mounts: so solutions area available for those who need it.
Where was this originally posted Jan?
Just to point out that none of the above applies to the TF.
Err....except the bottom bolts which should be replaced and torqued up correctly when fitting your Bilsteins.