Construct 2 is also a very viable engine, sorta similar to Stencyl, but different in a lot of ways - simple, powerful, and also updated regularly (roughly a weekly basis). Unity is more powerful than both, but doesn't exactly have the kind of 2D-focused toolsets that Stencyl and Construct 2 do. I prefer C2, myself, but it's something of a personal preference thing. Honey and Jack B Nimble, for example, use C2, while Ghost Song and Hollow Knight use Stencyl, though, really, any of these games could be made with either toolset.