Have you ever had to walk back microservices and re-adopt a more monolithic approach to your architecture? What spurred you to make that decision?

As the other comments here suggest, microservices - just like every other architecture approach - are not a panacea. Every design choice imposes a set of functional and non-functional characteristics: these characteristics are neither good nor bad, they are only appropriate to your situation or they are not.

Younger tech leaders often are too easily enamored of the latest tech trend: every significant design decision should be challenged by asking for the PROs and CONs.

Graham Berrisford (https://GrahamBerrisford.com/) has some excellent advice and reference content regarding all sorts of Architecture topics, including the trade-offs inherent in a microservices approach.

Yes we did it mainly because of concurrency issues with Microservices when we had unmanageable traffic. On top of that, tightly coupled assembly references, which cannot be simply implemented in Microservices also lead us to adapt Monolith.

Sometimes scaling vertically and horizontally a monolith application is much better than introducing overhead with Microservices, which makes managing it harder.

Yes, we once tried micro-services, turned out to be a bad idea from a performance perspective, which resulted in a slower user experience. It didn’t surprise us at all because there is always going to be some overhead, but we weren’t ready for us how much that overhead cost for user experience. It was also more complex for the engineering team to maintain. 

I really like Martin Fowler’s approach to this topic. He has an interesting article that isn’t exactly about moving back from microservices to a monolith, but it can give you some valuable insights on the subject.

https://martinfowler.com/bliki/MonolithFirst.html

Fowler is not against microservices, but he emphasizes that they should be adopted only when there is a real need and when the team has enough maturity to handle the complexity involved. Starting with a modular monolith provides a solid and flexible foundation, allowing the architecture to evolve as the system’s needs become clearer.