What did MongoDB not being ACID compliant before v4 really mean?

Question

I am not a database expert and have no formal computer science background, so bear with me. I want to know the kinds of real world negative things that can happen

Answer 1

As of MongoDB v4.0, multi-document ACID transactions are to be supported. Through snapshot isolation, transactions will provide a globally consistent view of data, and enforce all-or-nothing execution to maintain data integrity.

They feel like transactions from the relational world, e.g.:

with client.start_session() as s:
    s.start_transaction()
    try:
        collection.insert_one(doc1, session=s)
        collection.insert_one(doc2, session=s)
        s.commit_transaction()
    except Exception:
        s.abort_transaction()

See https://www.mongodb.com/blog/post/multi-document-transactions-in-mongodb