The front-end is still buggy and needs a designer's touch. And the real-time aspects wear thin at scale with all the notifications in a mismatch of languages showing up in the main feed. Lemmy is practically unusable without a personal feed filter, it seems that of the new reddit alternatives only retalk_[1] want to get this right.
If you do decide to run an instance of lemmy be sure to nuke the absurd word filter - the code is designed to make it as difficult as possible to change out. Thankfully some nice souls [2] are maintaining that anti-feature removal.
If we want to talk about Reddit alternatives, I would rather we look at ones which are open source, including Reddit itself until 2017 [1], the Mastodon Twitter-like web app [2], Discourse [3], Lemmy [4] (use a fork [5] if you can’t stand the slur filter), and the old school PHP discussions boards like PhpBB [6] and MyBB [7].
[1] https://github.com/reddit-archive/reddit
[2] https://github.com/mastodon/mastodon
[3] https://github.com/discourse/discourse
[4] https://github.com/LemmyNet/lemmy
[5] https://github.com/innereq/lenny