I rate this as a good card due to a scenario I often find myself in when playing :
Early game, I have money, I have 1 or more breakers in hand but no Faerie. Often I have a Special Order too. As crim you want early aggression, but also, with limited or no recursion you don't want to take Net damage and lose your precious breakers in hand. Or install the breakers then fear running because of Destroyers. In the past I would draw hoping to see a Faerie, often just drawing cards I don't want to see and discarding them (like Emergency Shutdown). I could Special Order for a Faerie but that never feels good.
In this case Mongoose is a filler breaker, it lets you poke and put pressure on early. All while stopping you from wasting your Faeries on low strength Sentries. When you then put down a Faerie you can break things ultra efficiently. In short, it fills a nice spot that needs filled!