[quote=@The Silver Paladin]The Moon Cell grail summoned Buddha. Your argument is invalid.
[/quote]

The Moon Cell is also a completely different Grail from the Fuyuki Grail, with different (read: basically nonexistent) restrictions on who can be summoned. It's not a good measuring stick for this, especially considering that goddamn Arcueid of all people got summoned by it, whilst apparently still alive at that.