Magic resist is either too strong or too weak...
When I play a Mage, I always feel that my build early game needed to have,
So I could kill my laner with 1 or 2 rotation of my combos, if they didn't build magic resist.
and if they build magic resist,
I would need at the very least 3 rotation.
If I were to go another build such as,
vs someone who would build
I would feel like I would not be able to kill the champion and only scratch them,
and when they would have this item for example
I would feel I would do 0 damage, and needed Void staff to even do damage to those champions.
So in conclusion,
Mages need Pen to snowball, if mages doesn't go Pen they would do no damage, Base Magic resist is too low, Stacking Magic resist is too strong.
is strong af against mr stackers and even squishies and