The vacuum formed risers were considered the best riser of their day.
Cost and availability was always a problem.
Shipping them safely was iffy as well.
I never heard any bad reports back from those that did purchase them.
Now, I use and recommend creating a five minute riser made with Morgan Super wool .
Cost is low, even with shipping.
Some safety concerns about breathing this after it has been heated over 1400F (Riser temps are known to be 1800-2000F)
However wearing a standard facemask or respirator a "used" 5 minute riser can be safely handled during a rebuild.
Prier to heating, Morgan super wool is a pleasure to work with.
Risers can also be hand built cheaply using forms with a fireclay and peralite mix .
Heavy firebrick riser can be wrapped with
superwool to insulate it.
High temp (2500F) + insulated firebricks also make a fine riser.
Watch for a soon to be available book by Paul Wheaton and Uncle Mud.
It talks in depth about different design risers and riser construction.
As well as all aspects of
RMH design.