I think there's truth to some of the "not ready" claims... and this is coming from someone who really tried to get into Lemmy, ended up creating their own instance (as demonstrated by my user handle).
A few issues I think Lemmy dev team really need to address ASAP, from least technical (thus affecting most users) to more technical (this affecting less users) are:
1 UX/Discoverability -- Finding communities are a huge pain in the backend right now, and with multiple communities on different instances serving same purpose (i.e.: !reddit@lemmy.ml and !reddit@lemmy.world). Sure, Reddit had same issues (the example I've heard is /r/meirl and /r/me_irl), but Reddit offered solution (multi on old reddit, community+community on new reddit). There must be a way to streamline it with meta-communities or lists on Lemmy such that the contents can be viewed in a unified fashion. I recommended !community@ (note the lack of domain) to streamline all of user's subscriptions with same name on different instances as an example; and perhaps we can use #list$user@lemmy.domain for users's maintained lists to unify !homelab@lemmy.ml, !datahoarder@lemmy.ml, !homelab@lemmy.world, etc.).
2 Trigger happy defederation hubs -- a certain instance has unceremoniously de-federated a couple of other larger instances. This is not the way, but here we are, with users on those instances not able to access the broader Fediverse, and vice versa. Until discoverability gets taken care of, it will be challenging for users to find a good home -- this leads to next point:
3 Authentication -- The Fediverse at large needs to separate authentication out from instances. Instances may provide their own authentication, fine, but there needs to be better way to authenticate against something else other than an entire new instance of Lemmy. The ActivityPub protocol has clear definitions on what is an actor, and users shouldn't need to deploy a Lemmy instance to identify themselves, separately from a Mastadon instance to identify themselves, separately from a... etc. This is because frankly...
4 Deployment of Lemmy is utter garbage. The official documentation's getting started guide gets users setup with an instance where the UI container cannot talk to public, but the lemmy backend can? Why bother shipping an nginx container if the backend will just expose itself to the whole wide net? Also, let's just pretend postgres container isn't open to the whole world with a basic password... Trying to get it up and running with Traefik was a pain, just do a quick Google and see how many people have asked and gave up, as well as how many different ways people have tried to go at it (something something xkcd 927; I've contributed to a new one of my own per linked post on top!), and the dev basically just straight up going 'we don't support traefik'... also, each approach is not without problems...
5 Federation is a removed. I am pretty proud of the way I've used override to not edit original docker compose, and locked my setup down a little. But, I'm not ready to have the instance open to the whole wide web without CloudFlare in front... but allegedly, Federation doesn't work with CloudFlare... why? Good luck trying to get to even a popular sub's scale without getting hit with DDOS when someone disagrees with something someone else posted.
There's many more problems, and I genuinely want Lemmy to work. But, Lemmy is, lack of better words, "not yet ready" for prime time. It is thrown into the spotlight with Mastadon (which feels a bit more mature, at least from reading the docs) because of bad leadership at mega techs... It will take a lot of work for Lemmy to evolve and mature, before it can be "ready" to really absorb the mass of Redditors leaving Reddit.