Even the core LineageOS and Graphine devs spend time disparaging it on reddit and twitter with comments like "It's not a very good app." and "Unfortunately Seedvault is still unreliable." Those are direct quotes.

I think it was back on LineageOS 16 that it was actually broken for the entire release. It wasn't even possible to do a basic backup at all.

Google for "site:reddit.com does seedvault even work" for dozens and dozens of examples.