It's an unsupported configuration and it's not tested.
The latter is a big problem, one of the extreme-OC guys (Buildzoid) who interacts frequently with the OEMs (as he is pushing their stuff to the limit and he frequently needs their help) has commented that AMD has a really bad problem with their BIOS teams. The AGESA firmware (the low-level code that the processor actually runs) is buggy as all hell at a firmware level and the OEMs are forced to patch around it in BIOS, but the AGESA firmware also has a massive problem with code churn, so these BIOS fixups basically stop working all the time. And the driver teams at a lot of OEMs are literally one person, so there isn't enough staffing there to test everything all the time. Long and short of it is: stuff breaks in AMD BIOSs, constantly, and they don't notice it.
This is obviously a huge problem when ECC is not an officially supported feature, because it means nobody is testing it! You might update your BIOS (as you frequently have to do with AMD machines) and suddenly ECC stops working, it might be running ECC in non-ECC mode and no longer correcting errors. Or it might have screwed up reporting them to the OS.
The server/workstation boards are the only ones you should be trusting Ryzen with ECC usage on.