Not answering for HashNode. But I have used both. Nginx has been there for a long time, its more robust, battle tested and being used for huge production work loads. The pain point I saw in Nginx is configuring it, I had to spend most of my time on configuration. If someone new had to alter the existing configs, they will take long time. I have heard people saying Nginx is faster than Caddy, of course it should be it's been there for 15 odd years.
On other hand caddy is light weight, less complex than Nginx, single binary file. It's also being rapidly adopted into production these days. The main advantage of caddy is that you don't have to focus on installation/configuration you can bring up an HTTPS server in 10 seconds with few lines of config file. On scaling part, I have not seen any hiccups with Caddy so far. But others opinion might vary.
Another advantage of caddy is you can embed it into your Golang code base, that is instead of using it as stand alone webserver you can import the webserver as a library. You can check my blog post on this:)
TLDR; if you want simpler, robust, lighter, out of the box HTTPS web server go for Caddy. If you want more robust, enterprisey webserver you can go for Nginx.