The recommendation for ECC memory is simply because you can’t be totally sure stuff won’t go corrupt with only the safety measures of a checksummed CoW filesystem; if the data can silently go corrupt in memory the data could still go bad before getting written out to disk or while sitting in the read cache. I wouldn’t really say that’s a downside of those filesystems, rather it’s simply a requirement if you really care about preventing data corruption. Even without ECC memory they’re still far less susceptible to data loss than conventional filesystems.
The recommendation for ECC memory is simply because you can’t be totally sure stuff won’t go corrupt with only the safety measures of a checksummed CoW filesystem; if the data can silently go corrupt in memory the data could still go bad before getting written out to disk or while sitting in the read cache. I wouldn’t really say that’s a downside of those filesystems, rather it’s simply a requirement if you really care about preventing data corruption. Even without ECC memory they’re still far less susceptible to data loss than conventional filesystems.