Wizards has helpfully told us how many of each rarity are in each frame: Theres a special case in the rares, Hidetsugu, who Ill get to in a moment
Another perfect example of this is Isshin, Two Heavens As One, which reads: "If a creature attacking causes a triggered ability of a permanent you control to trigger, that ability triggers an additional time." This is an incredible ability to have on a commander - and building a Commander deck around it is sure to be a blast - but in a Standard set it just feels off
Umbra armor has no effect if the enchanted creature is put into a graveyard for any other reason, such as if it's sacrificed, if the "legend rule" applies to it, or if its toughness is 0 or less
Meanwhile, Archmage of Runes not only draws cards the same way, but it also makes your spells cheaper, turning every cantrip into a little engine
Morbid Opportunist stands out as one of the few that trigger when tokens die, so its useful in the right context