In your scenario, I have to assume that my opponent played Shadowdancer or Shadow Sister Kelaino of their own free will knowing darn well that they could get punished by this “Moonforge Lancer” card. You know, that’s what counterplay is all about (coincidentally, the creators are those at COUNTERPLAY Games).
And as @whyb0t mentioned, it’s situational. Moonforge Lancer seems to be good against maybe 3 out of 12 generals (Zir’an, Lilitthe, Cassyva)That’s assuming they play cards that enable Moonforge Lancer’s effect, which if they understand the matchup, they won’t even play those cards.