Quote Originally Posted by ProsecutorGodot View Post
It's mostly aimed at the very specific scenario where the caster is going to be able to Teleport before your reaction attack goes off, the oft used "White Room Scenario" that people use to universally discredit this feat. While it does work that way with a straight reading, I'd be more willing to make exceptions if a player has invested in this feat, rather than when a player is trying to emulate this feat with clever words.

It generally does quite a bit in helping you slay mages, giving them disadvantage on a concentration check by itself is a powerful feature in my opinion. I don't personally think that it's a bad feat.
It is not a "very specific" scenario it is quite an extensive list of spells that ignore mage slayer completely.

Shocking Grasp
Darkness
Fog cloud
Hold Person
Invisibility
Greater Invisibility
Charm Person or any spell with the charm effect
Sleep
color spray
Hideous Laughter
Blindness
Misty Step
Suggestion maybe depending on what you suggest
Dominate person
power word stun
power word kill
Wall of stone
wall of force
Flesh to stone
Force Cage - the instant lose button to all melee characters
Polymorph other
Time Stop

Those are just off the top of my head and just spells on the wizard list in the PHB

A caster does not have to know that you have the feat to protect against it, any smart caster is going to assume their enemies have it and act accordingly.

If you are standing in their face with a melee weapon in your hand why would they cast fireball or lightning bolt when they could just cast darkness and leave you standing there, or force cage and take you out of the fight 100% of the time.