It's all kind of behaviour that aims to harm, hurt, or anger someone without being upfront about the intent.
If I hit you or insult you, that's not passive. If I say "some of us actually do the dishes around here" within earshot of someone that I believe doesn't do them often enough, that's passive aggressive.
Like most behaviors, it's not inherently bad... but I would argue it often is. There are better tools out there to solve problems.
You're right in that (in)effective would work here, but I did intend to also apply judgment. It often is not the most effective technique, but I also believe it is often unnecessarily hurtful and used as petty vengeance. I see it as an immature reaction from people who don't care about the receiver, or haven't developed other ways to communicate.
Take your shitty boss example : You're angry at them, and you want to express it. Anger is used to signal your desire to change a situation. If you're leaving, the only thing left to change is their behavior to future employees. If you use passive aggressive methods, you're either hurting them back for simple vengeance, in which case you just don't care about them as a human being because this hurting serves no purpose, or you're trying to change their future behavior. In that case, you will probably just paint yourself as an asshole, which will invalidate your position in their mind. It might sometimes work, but I would not consider it effective compared to being direct for example.