Hi, I have a HP T520. Had the very same problem.
I was about to do a clean install and restore, then came 15.2 and thought why not try before erasing.
15.2 update installed without problem, and also the slot a-b problem went away like it was never there.
Sounds good, more people having good results?
I read the changelog and it seems to be positive about the adm gpu issues.
I didt do any updates since the 15.1 debacle, should I do the core updates first or the OS update first, does it matter?
I don’t think it matters, but i was installing core updates during the slot A problem. IMO core has not much to do with OS, core is a container, OS is the container host.
I use a thin client Fujitsu Thin Client Futro S920 (AMD GX 415GA 4x 1,5GHz 8GB RAM) and it works fine with 15.2. I provided a log to sairon so he could fix the problem. With his support I could get 15.1 to run and when 15.2 was released the update was no problem at all 
My update to 15.2 went good! (HP T520)
I’m running on a RPi4.
First attempt at upgrade 15.1 to 15.2 after 4 hours all activity stopped and HA refusing to respond. I powered down and back on, after multiple restarts HA appeared still in 15.1.
Second attempt exactly as first except powering down and restart did nothing!
Did a clean install of 15.2 on new SD card and restored from cloud. Everything went smoothly and HA working without problems.
Lessons I had already learned (the hard way) - Do a backup before any upgrades. Download the emergency repair kit. Have a spare MicroSD card handy and . . . DON’T PANIC!
yesterday updated hp t630 HAOS from 14.2 to 15.2.
everything works ok 