I only recently acquires Miruna and I haven’t really tried her so I won’t base these comments on experience. Comparing her to Sweers, you pay one extra provision for one extra body, the ability to hit one point stronger target and no row lock. You loose targeting (which is a mixed blessing with defenders), and the effect requires consume rather than deploy (which is also a mixed blessing — while not ideal, Miruna could be used proactively). Given the dearth of three point targets, I would consider Miruna a real bargain in comparison. Of course, that doesn’t mean Miruna isn’t in need of a buff — just that Sweers is worse!
But what I really wanted to emphasize here is that Miruna is a great card in terms of game play. She can be very impactful, but achieving that impact requires some thought and care. Playing around that impact is also possible. And that makes the card very interesting from a strategic point of view.