For our use case, it just wasn't the right fit. In particular: 1) we needed more features/performance than Mongo's aggregation framework provided, 2) the mongod global lock was prohibitive for our write throughout (turning off fsync wasn't suitable), 3) schema migrations were difficult/impossible without transactional guarantees.

It seems like Mongo's addressed some of these concerns since we started building Heap.

Humm... No

They also outgrew PostgreSQL, and gave up on Redshift, is that a condemnation of them as well?