I’ve seen that as well, the default estimate for jsonb can seriously confuse the query planner. There is a patch in PG13 that addresses this as far as I understand, but I’m not familiar enough with PG internals to be sure I’m reading that right. I’ll be playing with this when PG13 is out, the jsonb feature is really useful, though I wouldn’t recommend to shove relational data into it. Many things are much, much harder to query inside jsonb than regular columns.

There are ways around the statistics issue in some cases, e.g. defining a functional index on a jsonb property will collect proper statistics.