Right now, NSFW-marked communities are by default(?) not shown by their home instance to non-logged-in users in the community list, and even if you go to them manually no posts are shown.
Fine, but they also aren't shown to logged in users on other home instances, unless somehow already federated over. If you go to the community's instance, it can't tell you are logged in, and if you go to your home instance you can't see a list of all communities on the other instance that might be available.
Also, older posts that are marked NSFW can't be gotten by anyone with an account anywhere other than the instance they were posted to. When you subscribe to a community on another instance it federates over a few posts, but to doesn't request and federate older posts as you try and page back through the archive. The normal solution is to view the old posts on the source instance, but if the community is marked NSFW the source instance won't let you read the archive there without a local account.
When first subscribing to a new instance, a handful of posts get backfill synced. I think it would make sense to also backfill the comments for those posts.