[minimeme]

No, I'm not a consultant. Maybe lead developer is the most accurate title for me:) By what criteria the biggest system do you mean? They certainly weren't toy projects, these are real systems in active use solving real world problems for thousands of users. The ES part of these systems is mainly implemented in .NET using excellent Marten DB as ES store on top of PostgreSQL. I would say that ES changes drastically how you model things and see business. It forces you to identify and define meaningful events. These are often actually something that also non programmers understand so it also improves creatly communication with your clients(or in DDD terms domain experts:)) as a by-product. Scaling also has not been a real issue, these systems can handle thousands of operations per second without exotic hardware and setups, all this mostly synchronously.

[minimeme]

And I must add, use the right tool for the job, there are many cases where ES is not a good fit. Also, if you choose to use ES in your project, you don't have to use ES for everything in that project. Same thing actually applies to asynchronous processing. If something in the system doesn't scale synchronously, that doesn't mean you now should do everything asynchronously.