If it's 2cm thick, then they should have put a layer of plywood underneath attached screwd to the top of the base cabinets. I haven't seen slats before be put on, but that doesn't mean that method is not correct, just that I haven't seen it before. But if it's 2cm thick, I don't believe there is enough support in between. If its 3 cm thick, then they could install it directly on top of the cabinets.
In any case, if you see gaps and the appliances don't fit right, then the job is bad regardless of the method used.