I think Resilent: Con is an A-tier pick.
Except I'm not actually trying to break records of how high my constitution saving throw was. Thats actually completely worthless to me.
In fact I dont care how high I roll in absolute numbers, I just want to roll high enough, as consistently as possible.
Which means Warcaster is clearly superior to Resilient: Constitution.
Very simple example, with Resilient: Constitution the chance for a critical fail is 5%, or one in 20, just like before.
With Warcaster the critical fail chance is lowered to one in 400, or 0.25%.
