I am not saying that microservices is a problem for Slack (at their scale it can make sense), but I am expressing my overall concern about smaller companies going down the same route when their scale or the problem being solved doesn't justify it and they end up having to deal with the (self-inflicted) problems of a massively distributed system with no major upside as well as my personal opinion of why I feel uneasy working on systems where I don't have a full overview of how it works and its potential failure modes.
When it comes to the benefits of microservices I am not sure whether those are all worthwhile considering the overhead and extra complexity of development on a microservices architecture.
In a monolithic application, most data and functions you might need are just a function call away and you typically have one or a handful of databases to interact with, often abstracted away by an ORM. In a microservices architecture, you suddenly need to worry about serialization, authentication and communication between services (and its failure modes, etc) and might require coordinated changes across several services, each of which might use a different language/framework and deployment process.
In terms of getting started, it has always been easier for me to work on a monolith where the codebase makes up for bad/no documentation because my IDE can resolve the majority of the symbols and allows me to see where the data I need lives and where it's being used. In a microservices architecture all of that goes out the window and you need to do a lot more manual "discovery" work searching through the documentation (if there is documentation, which is not a given) and manually figuring out the RPC calls because IDEs typically can't resolve cross-service communications.
Running a monolithic application locally is a lot easier than a microservices architecture. For the former you can typically get away with just a database and cache server all running natively. The latter pretty much imposes a container-based stack where you are now running 10 databases, caches, reverse proxies and everything involved around service discovery, which adds yet another layer of abstraction and makes you spend more time on this useless plumbing than actually getting work done and delivering business value.
In a monolithic architecture the devs that deal with it, have to deal with the program as a whole. So if something doesn't work, its their problem. Where as in a micro service architecture it can be easy to spin up a service and not know the systems that integrate with it.
The problem here is with documentation and understanding of architecture. Its just the nature of the beast that the monolithic dev knows how thing communicate with the monolithic program because he needs to know, in order to do his job. In this instance the problem isn't with micro services, its with the execution. And that execution is a very easy trap to fall into with micro services.
Microservices (like just about anything) can be implemented well or poorly. There's a reason we have sophisticated orchestration solutions like Kubernetes... it exists to tame large scale deployments that has sensible failover processes.
The benefits you get are services that can be scaled independently, deployments that only affect isolated pieces of code, horizontal scaling, dockerized environemnts, etc. All of these advantages should exist in well designed systems, but systems that have been executed hastily will likely have critical problems crop up at some point.