Many people build computers of varying complexities, but they usually use IC-s from the 4000 and 7400 series. These IC-s are mostly simple things, like NAND gates, adders and such (there are some complex ones). Making discrete equivalents of those IC-s is rather easy, especially with modern high speed low capacitance FET-s.
I am impressed by this computer because it is verbose and thus educational.
Building a complete computer the size of a room isn't easy at all, because the connections between the elements have to be active transmission lines, and Vcc and GND still have to be clean enough to avoid spurious switching.
There's a whole order of magnitude of extra design pain involved in building something that big, including LEDS in all the elements, and making it run fast enough not to be completely useless.
Most used, and thus cheapest, transistor BC 639 is 100MHz (or 50MHz, i had a beer). Cheapest MOSFET is much faster then that. The Megaprocesor is 20kHz and the creator clearly states that it is for educational purposes.
While on that topic; i will read the docs and watch the videos for educational purposes. ;)
I am impressed by this computer because it is verbose and thus educational.