What's your provider? I'm on Giganews and while retention is pretty high, old files (>2 years) almost always are a problem for me. Something is missing, and there are not enough of pars to fix. This happened so much that I no longer bother with anything above 2 years.
For the same cost as a Usenet provider and indexer (some indexers have a fee(s)), you could easily buy a little torrent seed box. I find torrent sites are much more organized in their content libraries and more reliable for quick, complete downloads - especially with a dedicated torrent box doing the work.
That is not to mention the added benefit of using remote storage so you don't have to have multiple TBs of drives around, and the opportunity to run a Plex server right from the box.
As a Usenet subscriber a couple years ago I have had the opposite experience you described: multiple failed downloads, poor retention, bad labeling (often foreign language content downloaded by mistake), and high barrier of entry for automation (relatively). I used DogNZB and combined multiple provider block accounts for redundancy.
Alternatively, the private tracker community is much larger and more active than the Usenet community in my experience. And they don't charge you for access. The seedbox situation outlined above can be just as automatic (autodl-irssi for one example) and often much faster (dedicated euroservers often burst to ~100Mbps to 1Gbps depending).
Always fullspeed, no waiting times, no problems fetching ~10 year old files. (if your providers retention rate is high enough) And great automation.
Just config your setup once and then just open Kodi each day and check what is new :)