But now we are talking about monsters immune to every spell worth using a LR the wizard can cast while people complain I am white rooming. So I'm not sure what is going on.
not every spell, but many monsters with legendaries are immune to many save or suck conditions and many legendary monsters do have magic resistance. Off the top of my head: Vampires, DemiLich, Lich, Solar .....
I believe all of them are either immune to frightened, poisioned and charmed or they have magic resistance.
Now give me 4 legendary monsters that are immune to stun ...... heck give me 2.
It is great at atripping LRs. Con is iften, if not always, a strong save on LR creatures. Using it on a stun strike is a good backup plan, making it even better at stripping LR.
Not "a stun", 4 stuns in one turn. Further Constitution is the strongest save on most monsters, but Constitution it is NOT the strongest save on most
legendary monsters. For the most part the only legendary monsters with the highest save in constitution are Dragons.
On most Legendary monsters Intelligence, Wisdom and Charisma are often higher than Constitution because legendary monsters often get proficiency bonuses to those (in addition to having a high score).
1st level slots are roughly as cheap as Ki points.
No they are not, especially at higher levels. A 6th level wizard gets 4 1st level slots per LONG Rest. A 6th level Monk gets 6 Ki points per SHORT rest. That makes 1st level slots about 3 times as expensive at 6th level and it goes up from there because the Monk keeps adding Ki.