I feel like Kestrel could be better. I've hit some issues in production (on Linux) there is no other choice.
It's not really that intuitive to use, compared to the offerings from languages like Go or Python. At least at that time it seemed like there was a lot of boiler plating code, that you wheren't really expected to understand, and the documentation really only go you to the point where things sort of worked, but you didn't really know why or how you where suppose to move on.
The only negative things I hear is when people assume it is meant to be internet facing and looking for out of the box load balancing or some such. For it's intended purpose behind a reverse proxy like nginx, I've had zero issues or missing features.
I've used flask, django, rails..and even Gorilla (go) none has loadbalancer.
I usually put memory limit to the docker instance with .net core. After doing this its been running great. stable, lightning fast!