The complication for me is that cards aren't very equal, and it's actually possible to build decks that assiduously ignore all of the conventions I mentioned here (my Pavel Maliki deck has 17 5 drops and an average MV of 4.20, while my Sunastian Falconer deck has a gobsmacking MV of 5.35, both are functional decks IMHO) if you simply run better cards that have relatively high MVs, and run enough of them that you will consistently have one or more to choose from
Spells with this kind of effect are becoming increasingly problematic in Magic - if seven mana additional turn spells like Nexus of Fate and Alrund's Epiphany can get banned, it's easy to see why Time Walk is so scary
While you can remove Drannith Magistrate, that requires you to actually draw a removal spell (in a 99-card deck), and its controller can't have a way to counter that removal
As sets rotate out of production, their foils gradually increase in price
Like the -3 ability which destroys creatures with mana value 3 or above