Knockdowns/stuns/silences/freezes on the player, and immunities that enemies have, are bad game design because they all have the same issue: they remove player choice.

The issue with knockdowns/stuns/freezes is that they remove the player's ability to do anything, at least how they work in most games. They make you take a timeout, essentially, and that's very unfun for the player. Essentially, it's removing your choice of what to do in the moment. You can't react, you can't flee, you can't fight, you just get to sit and wait or maybe press a button repeatedly just to wait a bit less. It is terrible game design that is wholly uninteresting, and it needs to be telegraphed nearly as hard as an instant-death move to be anything other than completely bad.

Silences do much the same thing in that they limit the player's ability to react and use their cool tools you just gave them. It's like handing a lumberjack a chainsaw and then saying "cool, now don't use it". It's not as bad as a stun, but it's pretty close.

Immunities for enemies are similar in that they limit player choice. You wanted to use cool X thing? Too bad, you literally can't win with that method. Resistances are fine (within reason, doing 1 damage is no different from 0 damage in a lot of games) because they allow a sufficiently-skilled player to still use a method they like (ideally), but immunities do nothing but kill build variety.