Quote Originally Posted by LudicSavant View Post
The point he's making is that you basically have to keep thinking "Sure, I could cast Cone of Cold, but is that better than 6 Stunning Strikes in this situation? And if so, how much better is it? Worth a whole subclass better?" As to the factors you mention...

1) Immunity to being Stunned is not common. There are 553 monsters of CR1+ in the MM, VGtM, and MToF. Of those, a measly 15 have stun immunity.
2) Magic resistance doesn't affect Stunning Strike.
3) Iterative probability means that 4 Con saves will often be difficult, even when 1 is not. This also helps chew through Legendary Resistance (particularly if your party is also using iterative saves).
High constitution is very common, though. And to put things in a different perspective, whenever the monster saves, the Ki point does nothing. Stun is a great debuff, don't get me wrong, but a DM isn't going to let their monsters get stunlocked. At least, I'm not. In practice, it's just not as good as it's being implied in my groups. As a DM, sure, you'll get a mook or maybe a hench but you're not going to be able to lock down a stunning strike on a good portion of enemies because all grounded fights with only melee combatants are boring, imo.

But then again, a monk casting an AoE spell means they're trying for AoE damage all at once. You're not getting 6 stunning strikes at one turn, it'll take at least two to even do that, and stunning strike doesn't add damage, just makes them more susceptible. Meanwhile, you're doing 6d8 damage to multiple creatures. The DMG suggests you'll land an average of 6 enemies at once with cone of cold. Realistically, that's 36d8 scattered across 6 different creatures.

The point is: High AoE damage can be invaluable and Cone of Cold delivers with a wide 6d8 damage with damage on-save and 60ft range. If you cast Cone of Cold on a single monster, you're a fool. But if your DM does nothing but single monster encounters at level 17, you probably didn't need to use stunning strike either.