Theyre also both pretty solid cards in general, so dont be afraid to play them alone
One thing I really hate facing down in Commander is a combination of cards that makes an opponents battlefield unassailable, usually involving cards like Avacyn, Angel of Hope
Y'shtola's last ability resolves before the spell that caused it to trigger
She is forced to mill two cards at the beginning of every turn shes out of the Command zone but that means you have a full Graveyard to pick from
You can follow it with your own 6/6 flying demon after unlocking Ritual Chamber, and at that point, youre draining your opponent for 2 every turn
Theres enough cost reduction and free spell-casting in Eternal formats that this might make the cut somewhere, but I wouldnt get too excited about my 3-mana counterspells lining up against Force of Will and friends in Legacy and beyond