Unfortunately all subreddit mirror requests to that bot are auto-approved, and there is no subreddit subscriber limit. That means that very large subreddits with already-active Lemmy communities get mirrored. That causes unnecessary duplication and a lot of noise due to frequent posts to the subreddits.
That said, I actually subscribe to a few of that bot’s communities for tiny, niche subreddits (like with 5k Reddit subscribers) that I followed. I use it because it will take time for Lemmy to reach adoption capable of sustaining those very specialized communities. It’s useful for links (less so for discussions.)