That said, I typically grab from site templates and don't do a lot of css/js manipulation myself. Though, if I did, I'd just set-up Grunt or Gulp to handle automatic regeneration of the final asset files.
For those asking why should you switch? Well, exactly, why should you? If you are happy with your current toolset and are productive using it, isn't it all about the final product anyway? Who cares what you use. But if you aren't happy with your toolset, or are just keeping an eye out for something that might be better ...
Hugo is known most for its speed. If you have a site with many, many pages that needs to be built quickly, strongly consider Hugo. It really is super fast. I'm actually considering building out a very large sample site with various engines to demonstrate just how fast it is. I'll try to find this post on Hacker News again and post if I ever get around to it.
Layout of content is extremely nice in Hugo. Basically, you just organize everything in directories how you want it to be rendered in the end. This can be overridden if necessary.
Documentation is also really nice. I'm actually about to build another client site and considered using Jekyll (you know, just to see), but found the documentation lacking compared to Hugo. It also seems to needlessly complicate things, but that's probably just personal opinion based on prior experience with Hugo.
There are a few downsides obviously. You get what you get, unless you want to add functionality and recompile. But now you have to maintain your own version.
As parent5446 mentioned, variables can be a bit confusing. They are not as idiot proof as I'd like them to be.