Prism
Explorer
Have a level 17 spellcaster put foresight on you and see what happens Spellcasters can often get in more damage via buffing the fighter types than dealing direct damage themselves.
Well our wizard wouldn't do anything as reckless as casting their only 9th level spell (foresight) at the beginning of the day (and hence doesn't take it), but lets just assume they did cast it for the final fight then it makes a difference of about 5 points of damage per attack with my character when using GMW. Not worth the spell in that case. It might do 60 - 80 points of damage over the fight (i'm assuming we have a cleric also casting bless). Not bad but not good enough for a 9th level spell. We typically use 9th level spells for gate and wish not some extra damage
As I said earlier, advantage does make this feat decent but its not that easy to come by for a fighter. A 17th level spell, using an attack to knock prone, invisibility (pretty useless at this level).