run your backend tests against this in memory and tests can be run in parallel instead of using a single real postgres instance

You get all the features of postgres.

I was shocked to discover how incredibly poorly IndexedDB works. I always thought it would be fast and snappy if a bit alien. But nope, it's incredibly bad!

Despite being a native feature to the browser it's incredibly slow, and the way it works in terms of fetching records based on non-primary keys forces you to either load your entire dataset into RAM at once or iterate though it record-by-record in a slow callback. Something as trivial as 10k records can bring your webapp to a crawl.