PostgreSQL JSONB: when does it make sense over a separate NoSQL store?

at work, we're currently running both postgresql and mongodb in our stack. postgres handles all the relational stuff, and mongodb is used for documents and more flexible schemas, primarily for user preferences and feature flags. i've been wondering if we could consolidate some of our mongodb use cases into postgresql's jsonb. it would simplify our infrastructure a bit, reduce operational overhead, and keep data within a single transactional boundary. however, i have concerns about jsonb's query performance for complex nested queries compared to mongodb, as well as its storage efficiency for very large json documents. has anyone here made a similar migration from mongodb to postgresql with heavy jsonb usage? what was your experience like? did you hit any unexpected performance cliffs or operational challenges?