Tweakie when you make 1 cube inside another or more, 5 sides are easy to do . the problems start whith the 6th side how to hold everything together, the more parts the cube has the plugs becomes more complicated , metal cube you can use epoxy ,wood ones you can not plus they are very easy to break , most of my wood cubes the walls are .062 thick and you must not forget the grain of the wood you can crush them with your hand easely now we come to the hard part, the tools .
Some of the tools are very small .079 width .300 long and must cut on three sides at the same time . If you use hss some time it will not make a single cube before it is worn down (high silica content) and the tool must not rub on any of it's faces because the wood will break or burn so i have to use carbide and grind them with diamond.Then comes the program .The progam can be short because the tool cuts on three side at the same time BUT you must have complete sense of what your tool is doing .It can be frustrating too look at the program without seeing the tool ,each tool is hand made .
Once the tool and the program are made , ANYONE can make a cube in a cube in a cube without any more knowledge( if he has the tool and the program).
So for me the skill is in making making the tool and the program.