I've never actually seen anyone take the mage slayer feat, it's usable so rarely, and unspectacular even when it does trigger. And of course it's close to completely useless if the DM uses MoM monsters, since NPC spellcasters no longer cast spells but use various special abilities that look like spells but aren't
It could do with a serious buff. Something like "When a creature attempts to cast a spell within 5 feet of you you can use your reaction to interrupt the casting with a melee attack. If the attack hits the caster must make a concentration save, or the spell fails", would be worthy of a feat.