I had an absolutely awful experience when I tried using Sqlite through the sqlx crate. The work was mostly cpu-bound, but I didn't mind blocking the event loop for a few seconds here and there, so I thought I would just run it on Tokio's worker threads. Big mistake, I ended up getting extremely low CPU utilization due to a quirk (Or less charitably, a bug) in Tokio's scheduler.

I eventually rewrote the whole thing with rusqlite, but apart from being non-async, I found the API much less ergonomic to work with. But at least Rayon worked exactly as I expected.