Blocks, like slabs will sink if the sub-base is not thick enough or compacted properly. With the slabs though, you'll see them cracking.
Most driveways are not designed to take the weight of anything other than a bog standard car. If buying a new build house you'll be advised that you cannot park a van on it for example.
To avoid the potential for the driveway sinking, you'd want somewhere in the region of 250-300mm good quality stone (Type 1).