My criticism is that the challenge doesn't really increase very regularly - once every 10 maps or so it's really, really hard, followed almost immediately by a map so easy it's boring. The other problem is that after a while it all gets a bit repetitive, since it's always the same rules and units, just different maps. That said I've played over half of the 100 maps and it still draws me back in. Did I mention it's free and has zero ads?
I was a huge fan of advance wars on the gba. There aren't many games like that on Android, but this one scratches the same itch despite being almost entirely different.