Pretty much this.

Mastodon had similar issues for a long while, but as it’s more mature than lemmy, export features now exist. Posts from a dead instance will exist on federated instances, but I think the biggest issue would be losing on what communities you subscribe to.

++ federation replication integrity tools, back-fill tools

Example from yesterday, comparing different instances on which comments actually made it to peer instances:

https://lemmy.ml/post/1239920 has 19 comments (community home)
https://lemmy.pro/post/1354 has 1 comment
https://sh.itjust.works/post/74015 has 9 comments
https://lemmy.world/post/103943 has 9 comments
https://programming.dev/post/29631 has 13 comments
https://beehaw.org/post/536574 has 7 comments
https://lemmy.haigner.me/post/8 has 6 comments (posting originated with user there)

Issue opened on Github: https://github.com/LemmyNet/lemmy/issues/3101