Originally Posted by
LudicSavant
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).