Lemmy.ml is generally unlikely to be defederated because it is the main instance, it's run by the developers who are in charge of the lemmy codebase. If the instance were to become particularly problematic for others then maybe it might happen, but generally the devs have been aware of that and they've moved their politics over to lemmygrad.ml.

Personally, I chose lemm.ee because it seemed like the main admin really knows his stuff. He's contributed to the codebase, been a key helper when things have gone wrong and generally had his instance working well when others were having loading issues. It's also quite widely federated with almost everyone, for better or worse. I actually made several accounts all over, then settled on this one - I have a lemmy.ml one also.

While it may be something of a flaw, it's also an advantage. Many people have chosen to run their own instance, some just for themselves as the sole user. Obviously this creates significant overhead for you, but it has the advantage of having complete control in which instances are federated.

In any case, the reality is that your accounts are all temporary and can be taken away at any time almost everywhere you go online. That doesn't mean it's particularly likely, and while it is perhaps more likely to happen on lemmy it's still not very likely.

FYI though most instances will publicly post about major defederations (although probably not for defederations of spam instances). Also, there's a mod log at the bottom of the web page, so you can see literally everything that moderators of that instance have done.