Longtime readers (Ha!) know that I focus heavily around control, in particular the question of how to address control as it pertains to solos. But control isn't just complicated (read: problematic) for solos.
Control adds a dynamic element to the game. But it can have an unbalancing element too. If you think of it like this, ignoring elements like -defense or vulnerability effects, fundamentally most control is about turn denial, or otherwise reducing the effectiveness of enemy turns. You use Icy Rays and you immobilize two orcs, if those orcs don't have crossbows, well, you've just negated two of their turns.
So, if we compare control to direct damage, how much "damage" is control worth? We can think in terms of denied monster rounds, and you can deny monster rounds through control or through damage (that is, by killing enemies that much faster). Put another way, if a controller stuns the entire enemy field for one round, and by virtue of having a controller instead of a striker, the fight takes one round longer (with no deviation in resources expended), the controller is equally effective as the striker.
In practice though, control is not consistent in how effective it is. If your go-to spell is Icy Rays, you as a controller are going to be vastly more powerful in encounters with melee only enemies than in encounters with non-melee only enemies. Of course, in ranged only enemies, you can immobilize enemies and then laugh as the strikers OA the crap out of them, but those encounters are the minority. Stuns are fairly consistent in effectiveness, however.
The bigger issue tends to be that controllers often (by the books) can deny more enemy turns than they miss out on denying through raw damage, at least at higher levels. Aside from that question, if encounters are balanced assuming a certain amount of enemy turn denial, then any action aside from enemy turn denial suddenly turns an encounter into a lethal scenario. It's basically "Press X to not die."
This is particuarly noticable when it comes to solos. Under traditional outlines of solos, a stun negates an entire round's worth of enemy actions. But let's say you're running one of those newfangled dragons that has a bonus action, and a stun (or daze) negates that action. Well, what's the incentive for the character to deny that bonus action?
Generally speaking, no matter how I design my encounters, one of the following occurs:
1.) Control just becomes too important.
2.) Control becomes impotent (due to blanket immunities)
Ironically, I often combine BOTH of these together, often by having control immune enemies with non-control immune add on creatures or the like.