In my opinion, I feel that the Cleric is too dependent on their tome. All other heroes have their talents that aren't dependent on a single item, while talents of the Cleric (mostly) depend on their tome; they cost charges to the tome and this makes them a bit vulnerable/weak when the tome runs out of charge, while other heroes' talents stay active, almost all the time.
(Almost all of their talents are started with "The Cleric can cast...", and this is kind of... funny).
Unless they're tier-3 or tier-4 talents, then I think it's not OK to rely on a single item like that.