
I think this is the best card in the game.
Drawing 2 cards for 1 is insane (Pot of Greed, anyone?). It doesn't need to succeed the test unlike Overpower and friends. It doesn't have hoops to jump through unlike Take Heart as you will want to be succeeding skill tests basically every turn (but failing a skill test without wasting an action is much more situational).
But on top of that, it even boosts the skill test by . Many times in my campaigns I have committed my "dead cards" like the third gun in hand or second copy of an ally for 1 matching icon, and this card provides that value on top of its main mode of draw 2. And if you're worried that removing the 0's will make the test worse - don't. Most decent tests will be helped by the way more than hindered by removing the 3 good tokens. E.g. if you're testing at +4 against a bag of 16 tokens and failing on -5 and , that's a 12.5% chance of failure. If you add Unrelenting to pass the -5 token and remove 3 others, you're at a 7.7% chance (would be 6.2% if you didn't remove 3 tokens). With bigger bags and more tokens to beat it only gets better.
And lastly, there is the flexibility of the other modes. You can help an important test by removing some nasty that adds doom or just securing that late game test by removing the -8. Or you can throw in Unrelenting on a failed test just to draw 2 like you would Take Heart.
There are only two downsides I see with this card.
One, the deck archetype that likes cycling the deck a lot with cards like Unrelenting gets countered by certain basic weaknesses like Through the Gates, Amnesia or even Chronophobia. Some of these can be countered with Ikiaq, Deny Existence, Peter Sylvestre etc. if you have access to these cards.
And two, the expert difficulty usually only has 2 tokens that let you draw with Unrelenting. I've been testing the usage of Keep Faith to enable Unrelenting in this case, and I think it's still playable in this state although the nerf of needing to wait for your other combo piece is definitely noticeable.