How many Lisp implementations follow the HyperSpec? I'm no Lisp expert but from what I've seen the object system can vary quite a bit depending on the exact implementation.
free: ABCL, CLISP, CCL, CMUCL, ECL, SBCL; commercial: AllegroCL, LispWorks. There are more.
I've made a diagram a few years back that shows the family with ancestors and sub-branches including these that are not complaint: https://ecl.common-lisp.dev/static/quarterly/img/vol4/all-hi...
Common Lisp strength is its standard; many programs developed on one implementation will work on another one. And standard includes CLOS.
As far as I know, scheme had a bit of problems with that the standard was too small, and there were numerous implementations of the same functionality that were different; the issue with fragmentation was also problematic in pre-common lisp times - that was one of reasons why there was a grant to create the standard - to unify numerous implementations.