as someone who hasn't implemented transactions in microservices before, I have only seen the bank transfer example and it seems adequate and easy to understand - I wouldn't know what limitations it has
It’s fake. As a result, you can’t actually evaluate the engineering trade offs of the proposed solution.
Here’s the thing: do you even need transactions (in the sense of ACID transactions that are resolved heuristically, as described in this article) between your microservices? Likely not.
And what are the implications of having to wait around for PGSQL and Mongo negotiate a transaction between a third data store? Probably blocking and latency and all the associated problems which will lead to … not use transaction across service boundaries.
Both parties misunderstanding the other is a common trope in movies and novels for a reason.
The unreliable messaging links (internal mail) mean that resilience against missed messages and guards against not making progress must be built into the business process instead of an infrastructure layer.