jointhefediverse.net seems to be a commonly linked resource for directing people to join the Fediverse.
Curiously, it does not list Lemmy under the list of Reddit alternatives. Their GitHub README explains why.
Previous relevant discussion: https://lemmy.ml/post/78808
Yeah, I don’t expect it to scale well. Certainly not as well as Rust.
In terms of incoming federation, PieFed sites are dealing with as much activity as any general Lemmy instance. It’s not happened yet, but I suppose it’s possible that problems will become apparent if the amount of local users gets over a certain size. A limit on the amount of users per instance isn’t necessarily a bad thing though (it’s cheap, and hopefully easy enough, for someone to spin up another one).