Sorry for the late reply, this thread was just brought to my attention.
As the person who deals with every bug reported to Fedora’s kernel bugzilla, you would be quite surprised. There is not much of an increase in hardware support regressions in rebases vs stable updates. The current stable update cycles tend to run in the hundreds of patches backported. Also, as vendors have started moving functionality out of drivers and into firmware, linux-firmware updates seem to introduce a large number of regressions as well. Of course they get reported as kernel bugs most often, because users don’t use a new firmware until they rebuild the initramfs and the bug is exposed in the new kernel, but doesn’t appear in the previous kernel because the initramfs for it still contains the old firmware.
The purpose of test weeks isn’t so much what you think. Rarely do we see some massive regression in hardware support. Most of that is caught in rawhide testing. What we are actually looking for in test weeks is breaks in userspace on older releases because the kernel has dropped or changed something which works fine on rawhide or even the current release, but might not work so well on an older supported Fedora release.