They changed to Unity for the iOS version.
The first & very succesful PC version was done with gamestudio and was featured on steam.
But now they are working with Unity, because it works for them.
That´s the only thing what counts imo: Use whatever works for you.
For instance, I wouldn´t use GS or Unity for a 2D adventure. I would go for other solutions, that are build towards my goal.
For prototyping ideas I use Gamestudio quite often, because I´m used to it. But to prototype a physics based game, I use unity for example.
The world is not black & white. You don´t get vanished in nil or end up in hell if you use different engines and tools.
No big bad geek will come and kill you. Believe me.
