I don't have any personal experience building a machine for surfboards, but can tell you that the ones I've seen are VERY large. Much larger than what is contemplated in these types of machines. The one I saw had 12' of travel by 3'. This is almost an order of magnitude larger than the basic Rockcliff machine. The components used in Rockcliff plans would be very hard to find in that size. If you could find them in that size they would likely sag and lack the rigidity you need over that distance. Finally, using all-thread as the drive mechanism would mean that carving a surfboard blank would probably take hours upon hours in the machine. I've seen professional shapers shape a board by hand in about 90 minutes.
Unless you are pumping out dozens and dozens of identical boards, I don't think CNC is your friend shaping boards. If you are making dozens and dozens, then you probably can afford one of the professional surfboard specific machines out there. Also, with Clark Foam gone many people are switching to EPS. It is much cheaper than PU, but usually comes in square blocks (at least if you are not buying from a "blanks" dealer.) Blanks are being carved using hot wire. Routing off all that excess foam is going to take forever. Hot wire and a template makes pretty short work of rough shaping a board. There is no "top skin" to worry about so you can carve the shape anywhere in the blank. Hot wire, Dragon rasp, and coarse sand paper and you have a board shaped in a fraction of the time a CNC machine would.