From RedHat's announcement blog and Centos website: CentOS Stream "a continuously delivered distribution that tracks just ahead of Red Hat Enterprise Linux".
And indeed, I have read that all packages currently going into Stream 8 follow exactly the same testing and validation procedures than for entering Redhat Proper.
But of course, this holds true only for released RedHat versions. Meaning that Stream 8, is currently just ahead of RedHat 8.5 and will soon start to incorporate changes planned for 8.6 (so nothing like a "bleeding-edge test environment to test new changes" as you put it).
As for Stream 9 which was release recently, it is indeed the stabilisation socle based on fedora 34 that will eventually become RedHat 9. Once RedHat 9 released, Stream 9 branch will be stabilized and once again be "just ahead" of the impending RedHat 9 rpms.
In any case, I am not sure that hospitals should be running/updating self supported OS on surgical equipments without proper testing anyway.