Keep in mind that nano's name is derived from the fact it's a clone of pico, and things might start to make more sense. GNU nano's name is a play on words, not a statement about its size.
Nano has a surprising amount of features. I actually use it as my main text editor, and I rarely find myself lacking anything I could possibly want. Off the top of my head, it has the ability to run 3rd party code, a file browser, autoindenting, linting, macros, and a rudimentary autocomplete. A lot of it's reputation for being weak comes from bad defaults, which can be quickly fixed by ricing your nanorc file.
To be fair, it also has more features than I expected, e.g. syntax highlighting. And I’m sure it runs on an obscene number of platforms.