I have a 3970X with ECC RAM and got error correction notifications in my Linux logs when I tweaked RAM timings too tight. Note that memtest86 doesn't know about ECC on Ryzen, so you may get unnotified error corrections happening if you use that.
BTW, Century Micro has the only unbuffered ECC modules at 3200MHz native speed (at least that was the case on the 39x0X release day). I don't know if they can be sourced outside Japan, though.
Fun story, I actually botched my order and got 2666MHz ones... but on closer inspection, it turned out the chips on the modules were actually native 3200MHz ones. With the SPD EEPROM saying they are 2666MHz. So I ended up overclocking them at their actual native speed. And I tweaked the timings to be a little shorter than what the 3200MHz modules were advertized for.