At first sight, Miruvor looks like a poor man's Unexpected Courage. It allows you to ready a hero, yes, but the cost is discarding Miruvor. It does come with the option of returning it to the top of your deck to reuse it. Moreover, both cards belong to the same sphere. Obviously, the fact you have to discard Miruvor, makes it worse than Unexpected Courage.

Is this assessment entirely fair though? Miruvor also allows you to switch resources in dual- or multi-sphere decks and can give a hero a modest boost. Does this kind of flexibility redeem it's main flaw? Perhaps, if it wasn't for Miruvor's biggest flaw, one which isn't immediately obvious: Miruvor gives you the option to return it to the top of your deck. Why is this bad? Surely, it makes the card incredibly useful because it allows for the potential infinite reuse of the card. However, if you for this trap, be prepared to lose a lot of momentum in your game, because Miruvor will slow down your card draw. If you don't watch out, you will draw too few new cards -- or not at all.

What are ways of side-stepping this issue? Of course, one option is card draw, and Lore will give you plenty options for that, for example with Gléowine or Daeron's Runes. However, you need to consider whether redrawing Miruvor is worth it over getting more new cards from your deck. After all, you could've drawn another card instead of redrawing Miruvor again and again. Another option is hero Gandalf, which, if you time it wisely, allows you to replay it, but only in the Planning Phase of course. Fact is, even hero Gandalf gives you card-draw in a way, and it looks like every method of card draw will have the same issue: is slowing down to recycle Miruvor worth it?

All in all, Miruvor is a tricky card to use. It gives you potential action advantage and the flexibility for reuse or other options in a pinch -- but it'll came at the cost of a Siren's call to return it to your deck, slowing you down with it's lure.