I have a slightly older system with 128 GB of UDIMM DDR4 over four sticks. Ran just fine for quite a while but then I started having mysterious system freezes. Later discovered I had somehow disabled ECC error reporting in my system log on linux... once that was turned back on, oh, I see notices of recoverable errors. I finally found a repeatable way to trigger a freeze with a memory stress testing tool and that was from an unrecoverable error. I couldn't narrow the problem down to a single stick or RAM channel, it seemed to only happen if all 4 slots were occupied, but I eventually figured out that if I just lowered the RAM speed from standard 3200 MHz to the next officially supported (by the sticks) step of 2933 MHz, everything was fine again and no more ECC errors, recoverable or not. Been running like that since.

Last winter I was helping someone put together a new gaming machine... it was so frustrating running into the fake ecc marketing for DDR5 that you mention. The motherboard situation for whether they support it or not, or whether a bios update added support or then removed it or added it back or not, was also really sad. And even worse IMO is that you can't actually max out 4 slots on the top tier mobos unless you're willing to accept a huge drop in RAM speed. Leads to ugly 48 GB sized sticks and limiting to two of them... In the end we didn't go with ECC for that someone, but I was pretty disappointed about it. I'm hoping the next gen will be better, for my own setup running ZFS and such I'm not going to give up ECC.