I'd say it's not picking a server which is complicated. I think people don't really get the concept of federalized social networks. Honestly, I myself had some minor issues understanding how this works and I was studying software engineering in university. Hopefully this concept lifts off hard in the following years, because I think it's a future of the internet.
Seems like they're thinking exactly that. However, I expect many communities (especially niche communities) to revolt and start spamming unrelated content and straight up porn (or worse -- gore). The current mechanism is to replace the mods with supermods (aka the ones who moderate over 100 communities or something) but I wonder if they will be able to effectively moderate that shit without going private.
It's quite hard to guess what will damage reddit the most. I hope that many niche communities collapse and migrate, but the issue also lies in mainstream communities. One thing is NonCredibleDefence moving. Another thing is stuff like r/manga (which went for a blackout but didn't do much beside it).
Lemmy is good, I hope more people move here, but so far it's gonna be reliant on migrants from reddit and possibly twitter, which is not exactly a good plan