Edit: obligatory explanation (thanks mods for squaring me away)…
What you see via the UI isn’t “all that exists”. Unlike Reddit, where everything is a black box, there are a lot more eyeballs who can see “under the hood”. Any instance admin, proper or rogue, gets a ton of information that users won’t normally see. The attached example demonstrates that while users will only see upvote/downvote tallies, admins can see who actually performed those actions.
Edit: To clarify, not just YOUR instance admin gets this info. This is ANY instance admin across the Fediverse.
What account data are you referencing?
I mostly mean username and password credentials, primarily the storage and handling of passwords. Most people don’t take cybersecurity seriously and use the same username and passwords for every site they log into. Someone steals your Lemmy data, and they can try it on every social media and gain access to everything. Now, I am not one of those people, and my Lemmy account credentials are unique to Lemmy only, but imagine if someone joined and used the same credentials they always do, including for their bank login. There is where the concern is.