Check this out: https://lemmyverse.net/communities?order=subscribers

Not an expert on Lemmy, but:

So if a instance goes down, does that mean all accounts would be gone?

Essentially, yes, all accounts of that instance would be gone.

Would people lose all post/comments regardless in which community/instance they posted?

No, posts/comments on other instances would still be available.

Unfortunately, my previous instance has had some issues with stability and database corruption repeatedly. Lost posts, subscriptions etc. I am here now due to the similar federation policy, which I believe is the right one. The performance is also great. Very promising!