Communities is definitely limited. You can't sort them, you can't filter them (particularly by instance when looking at All).
The best way to discover new comms seems to be using Lemmyverse, apparently you can set it to your own instance so that links come up in your instance, where you're logged in and can subscribe.
Filtering would be nice, particularly by instance. I've had a few instances where I've opened the instance directly, saw a few I want to subscribe to, but then had to search for them one by one in my own instance.
However you can set your home instance in Lemmyverse and get links for each instance via your own, where you can subscribe.