Great concept, but a lot of the mechanics can be very infuriating when you don't know how they work, and there's no tutorial to tell you that they work like that.
One time, I marked a space as a mine, but didn't realize mines behind locked tiles count towards numbers, so I lost the game.
Also, playing in hard doesn't seem to give more coins than medium because there's less space for coins to appear in, so it's overall better to play in medium over hard.