My impression - the people who use it historically (and I've met a few) aren't the companies that typically proponent open source software and the like. They are "closed source" shops (e.g. finance, insurance, etc). Even I am not willing to post on more than a throwaway account. I'm currently using it for large scale production systems in a very large public company powering a large data volume product with very high peaks of customer traffic and it works a treat. We decided to try C# 9 after some F# because "higher management" - compared to F# the dev's have found it verbose and painful still, despite the new features.
The productivity benefits are small and sprinkled - I don't personally think there isn't one "killer feature/app". It isn't just one thing, its little things that add up. Given the team jumped from other ecosystems (e.g. Go/Node/etc) they found F# easier to approach than C#. This is the perspective of the team I run and it comes up in PR comments (I do less code writing these days). Comments like "don't have to do this in F#", or "we need a framework for this because C#" are known to occur. Easier unit test writing, less dep injection headaches, concise function passing, easier inlining of math for perf, easy mocking/stubbing, unions, etc etc.
The big weakness to me is that the people that use it typically don't want to flaunt it, and that means good mentoring, the best/simple patterns to use, etc and management buy-in are not really public. Communities that you can join are not into large scale apps, meaning good scalable patterns and lessons learnt are hard to find.