First of all, I assume that everyone knows that Ngroups - like probably about 99% of usenet "companies" [i.e., websites] out there - is a reseller, not a back-end provider. Therefore, the files you download actually come from a server farm owned and run by someone else.
Ngroups resells access to Highwinds/Usenetserver server farms, which is probably the most widely resold backend service provider.
It seems what usually happens is that the older files stored on the servers get corrupted somehow, and rather than just leaving files with a lot of incompletes, Highwinds will delete all those old files. I've seen retention suddenly drop more than 10 days at a time - across all groups.
So this is not just Ngroups, but all the resellers that use those same backend servers.
In contrast, Astraweb owns its server farm, and does not resell its service. But Astraweb is still having severe problems in the aftermath of a major disaster on both their US and EU server farms, so until they fix their problms, I would not recommend anyone buying an Astraweb account right now.
regarding encrypted SSL: I've noticed lately that the Ngroups/Usenet-News SSL server seems to be a lot slower per thread and requires a lot more connections to max-out my line. Even the Amsterdam server across the ocean is much faster per connection.