I think that are some challenges that are inherent to federated models, and I do also think there are challenges that mastodon has that lemmy/kbin can potentially avoid

Common challenges

How federation works is confusing to the average non-technical user. This increases the barrier to entry
There is an inherent tension between instance best practices (keeping the size manageable, having proactive moderation, not federating with nazis) and rapidly scaling federated networks to much larger numbers of users. Thus even in the best case scenario, onboarding a mass "migration" is hard to do well

Key differences between mastodon and lemmy/kbin

Mastodon is basically hostile to search and discoverability. This is an important feature for most of the people who really enjoy using Mastodon, but it's a huge downside for a lot of folks who thrived on twitter. But Lemmy has search! Kind of janky, but it's there.
It's hard to get people to use tags. Twitter clout weirdos simply made tags too uncool for most people to willingly use them anymore. And what little discoverability there is on mastodon relies on tags. Lemmy doesn't have this problem because communities/subreddits never acquired the stigma that tags have.

Cultural stuff which is very tbd on here

Mastodon has a huge problem with passive aggressive white techies and their micro- and macro-aggressions against people of color, which has flared up repeatedly and made it harder for key communities to flourish on Mastodon. Unclear how this kind of thing is going to play out on lemmy. Seems at a glance like there are fewer passive aggressive liberals but way more outright edgelords.
There's a large cleavage between two different conceptions of how federation and defederation should work, which in Mastodon has resulted in people being split between a few huge, poorly moderated mainstream instances that are kind of tedious to be on, and many small curated instances that are pleasant to be on but have complex siloed defederation politics, and also the nazi/spammer gab cesspool.
Mastodon has apolitical techbro development but a huge lefty userbase; lemmy has hard-left development but a huge apolitical techbro userbase. (This is extremely funny to me.)