Fair. Though it's still a good tactic for many.
It is extremely situational.
I am not saying the CME is bad, but it is extremely situational at the level you could do this, in a large part due to the short range and the kind of abilities and legendary actions a single boss creature is going to have and that makes it not OP.
And all rangers now get Blindsight at level 18. So he can join in the darkness party.
5 Dex + 12 Expertise in stealth + 10 PWT = +27 stealth.
So yea, automatic wins.
But the monster you are fighting can see right through it too.
And you have 4 party members in your example and you have to pass it with all of them (or half of them if you use the group check rules)
And you need to be obscured to even get a check, which you are not even inside the darkness.
So stand up.
Prone isn't a problem.
Except he is 55 feet away and it costs half your movement to stand up.
It can't fly as a reaction, and has +0 initiative.
It is not a reaction. It can fly as a legendary action between the Monk's turn and your turn while also knocking you prone and damaging you with the same Legendary action (and causing a concentration save). This is before you even get a turn. Most boss enemies you are going to fight at this level are going to have legendary actions and being within 15 feet is going to hurt even if they can't move away.
The boss I was fighting tonight had a DC23 dex save legendary action that was an AOE that does around 40 damage. That means sure - beat her initiative and get drug by another PC to within 15 feet and then you get to save and get to save again for concentration. You might make it or you might have lost your 9th level slot before doing anything with it (and also lost the dragging guys action) .... and she can do that between every single turn. Oh and if it gets to her turn she can bonus action AOE stun you on a 23 save and finally all attacks against her are at disadvantage (this is a from a published WOTC campaign).
Most bosses are going to get these kinds of things at this level. That is why I asked to provide an example of monsters where this will likely to work. There are not many. When you find one it is going to be truely OP, but there are other spells equally as OP against certain monster (Forcecage anyone).
It is possible (though unlikely) it will get a turn to go and blast you. But that's what Mind Sharpener is for. And the monk can ready a dash when needed.
If the Monk drags you within 15 feet it will damage you. It is a 25 DC dex save for it not to damage and the initiative count does not matter. It can be worse though. If another PC goes between you and the Monk it can attack you after the Monk's turn (likely hitting and causing a concentration save) and then use another legendary to prone you damage you and move 40 feet away after the other PCs turn.
Moreover breathing on the caster will likely end his concentration on CME, even with absorb elements.
The dragons best bet is to hit the Artificer to get Haste to drop, though she has Flash of Genius + Con saves + Warcaster.
Maybe he will do that on his turn, but it is not his turn it is the CME caster's turn and he is prone and 55 feet away from the dragon at the start of his turn, had to make a concentration save, and the Dragon is flying and up in the air unless this whiteroom is in a situational scenario where he can't fly.
3 other characters who still have all their actions and the majority of their spells / resources.
The Monk used his action to drag you up to the Dragon I thought? In any case sure, but are we tallking about 4 20th level PCs bringing down an Ancient Red Dragon (fairly easy) or about CME being way OP against an ancient Red Dragon (I don't see that at all)?
It is deadly for a party of 4. And I am using a level 8 spell. Multi classed to much to get 9th.
And this party at level 16 still has a solid chance of killing the dragon before it goes.
The party is level 20, not 16 in the example you game and they have a chance of killing the Dragon before its turn, but not before it "goes" because it "goes" with a Legendary action after the first person in initiative regardless of who that is.
They do have a small chance of killing the dragon but almost no chance of cashing in on CME and if 2 PCs use actions to try to land CME they have almost no chance of killing the Dragon either.
And keep in mind, CME is a 10 minute spell. So you can possibly get multiple encounters out of it. Just a EB + quicken EB should be able to take out half the enemies.
If you can get within 15 feet of an enemy it will be useful against and manage to hang on to concentration sure .... but that is not going to happen a lot.