Its generally best to cast it on your turn, but the flexibility of playing it on your opponents turn is nice, and youll have plenty of options to discard since you see four new cards for 4 mana
My favorite example of this style of deck was Abzan Constellation, one of my favorite decks of all time
Each booster, whether thats a set, collector, or draft booster, contains a Multiverse Legends card
Having one of either piece makes it less attractive, but if you have two or more copies of both, its time to build around it
It can eventually become unblockable of course, but paying 4 to make your 1/1 unblockable is going to feel really bad sometimes, especially because the Thief is so darn easy to kill
It seems likely that Shinka, the Bloodsoaked Keep will stabilize somewhere around $30