As the tiles are all the same you would expect the grouting to be fairly even. Tiles are allowed to vary slightly in size from each other which can give a slightly uneven grout width, but the tiler normally picks this up if there is any noticeable variation and advises on a slightly wider joint width. I assume your tiler did not do this. The grout should all be finished to a point level with the top of the joint gap (note some tiles have a cushioned edge, the grout should finish at the top of the joint gap, not the top of the cushion edge).
From the picture you sent the grouting joint appears to vary excessively and the surface of the grout appears to be higher than it should be in areas.
Cracked grout is often caused by movement, however your comment that the tiler applied a second cost makes me wonder if the grout has failed to adhere to the old grout layer. Did he apply a very wet second coat of grout? Which grout was used?
From the picture you sent the grouting joint appears to vary excessively and the surface of the grout appears to be higher than it should be in areas.
Cracked grout is often caused by movement, however your comment that the tiler applied a second cost makes me wonder if the grout has failed to adhere to the old grout layer. Did he apply a very wet second coat of grout? Which grout was used?