I would guess compared to Militia it is easy to get to strong. As it is, it is a decent opener, and against or with BM, it has the chance of bringing important $5,$6 or $8 hands below this treshold during mostly the whole game. Militia is mostly the same against BM.
Now if it would also target other treasures, you would mostly punish bad hands. The less money you have in hand, the higher the likelyhood that you lose $2 or even $3, especially in late game when you are full of green. Now consider you play engine against BM, and you manage to play 3 Cutpurses each turn, which is not really unlikely to be supported by a board. This surely blocks your opponent from Provinces, and very likely will leave them with at most $3 in hand, and you got all the time you need to finish your strong engine, without the danger of your opponent getting to many points.
So with this, I think you could at most let it attack 1 time per turn, so limit it with >4 cards in hand. I think this would be the worse card.