Unity is really great for isometric games. We combine 2D and 3D content + awesome special effects. Game works with unity Flash build too (before it was closed). And it haves low system requirements (working on simple android devices with minimal screen resolution too).
P.S: we are selling framework of this game, so you may create your own 2d/3d farm or any isometric game for mobile and social networks
There is in-game payments (you can buy rubies). But its very easy (crystals required for rare decorations and speed-up) and you can play without any donations.