Similar to CPUs, where many arrays have spare yield capacity, even whole cores can get disabled (and possibly sold in a different bin). DRAM stores redundant electrons in capacitors to patch it up and boost yields. Everything in reliability is a spectrum.
"ECC" does not give you fully reliable RAM. UEs are still be observed.
What's the chance of fail? If you have one device that achieves equal performance with less reliable cells and redundancy to another device that uses more reliable cells without redundancy, it's not really any different.
NAND is horribly flaky, cell errors are a matter of course. You could buy boutique NOR or SLC NAND or something if you want really good cells. You wouldn't though, because it would be ruinously expensive, but also it would not really give you a result that an SSD with ECC can't achieve.
"ECC" does not give you fully reliable RAM. UEs are still be observed.
What's the chance of fail? If you have one device that achieves equal performance with less reliable cells and redundancy to another device that uses more reliable cells without redundancy, it's not really any different.
NAND is horribly flaky, cell errors are a matter of course. You could buy boutique NOR or SLC NAND or something if you want really good cells. You wouldn't though, because it would be ruinously expensive, but also it would not really give you a result that an SSD with ECC can't achieve.