Yes, we leveraged Clojure a lot. PostgreSQL has far more advanced indexing and querying capabilities, particularly around too big to fit into RAM data, than what I'd want to waste time on building myself (if my team could even do it). The D in ACID is pretty important btw, and being able to offload the replication/backups/failover to AWS (using RDS) was a huge win operationally.

Clojure's HoneySQL is the best way to programatically interact with a database I have found, and it's not particularly close. It provides nice data structures to work with (that can even be schemed or speced) and combined with clojure makes writing complicated logic a breeze. It is easily extensible to new SQL functions, clauses, and operators, even vendor-specific stuff (we used a lot of PostgreSQL specific features around function-based indexing). LINQ doesn't compare to the flexibility here provided by data structures and a potentially just in time SQL compiler. No ORM does. As an example, the user might be submitting ajax requests to my backend, filtering some data by adding more filters or other clauses to my query. What I store in the session or in the database is fundamentally just (pr-str my-query). When the user comes back and wants to continue modifying it, I (read-string my-query-str) from my store. Then I just keep assoc'ing, filtering, and manipulating the data. If they decide this is a nice query, I just save it. It can become an alert, a report, etc. The only machinery is basic clojure data structure manipulation, basic print/reading, and the power of compiling data structures to SQL.

As far as operating on streaming data via Kafka (raw) or with Kafka + Onyx, I suspect you've never actually done this in Clojure. It's a breeze, quick to write and performant. I've yet to meet someone who has worked with Onyx who wasn't blown away by how simple it is to use and how quickly you can evolve very complicated data flow graphs. It provides reasonably low latency, customizable batching, and I can still write straight-forward and simple Clojure. You don't really deal with state or multithreading when dealing with Onyx (Onyx handles that for you and you can declare the parallelism behavior you're looking for). When I consumed Kafka directly, I didn't have many problems that get bogged down by state. We'd keep some local state, but the system was designed to either run in batch to consume some entire time interval or to be able to be kill -9'd at any point, in which case the data was designed with idempotence in mind (such that we can handle/detect processing data more than once).

I selected and championed Clojure because it was a dynamic language (very similar to python or ruby) that had access to battle-tested JVM libraries. I selected it over python and ruby, because it made choices that resulted in simpler programs that have far better locality when I or a co-worker must understand and modify a program in the future. The emphasis on immutable data and functional programming allows nice straight line programming where all I need to know to reason about the code is in the function arguments. There aren't as many surprises. The reason I've stayed with Clojure is the community focus on simplicity and the stability of my code. The code I wrote 8-9 years ago works just fine today. That's not a huge brag (cough Fortran cough Common Lisp), but it's refreshing when I'm looking at how quickly evolving languages like Rust and Python.