Refia is great, but she needs duel cast to shine. I think we're past the point now where she fits every situation though, and Yshtola is sometimes more suitable.
Weird, never had mana problems with Refia and now we even have the vestment of mind. I don't really like Yshtola, her lack of full life annoys me. With Refia, often she got me out of trouble with dual casting full life and using Cecil to dual cast curaja as the backup healer.
Doesn't matter, once tilith 6* is out they'll both be benched.