It's easy to criticize the work now after so much else has been done since. But at the time it really made an impression. For myself it started me thinking in more critical terms in regards to how things could be laid out and what needed to be shown to make something more credible.
I have to say his drawings of the shuttlecraft in his Star Fleet Technical Manual
started to bother almost me right off. He drew it as basically a hollow box with no room whatsoever for mechanicals behind the walls or under the deck. There was also no aft cabin. The interior bore next to no resemblance to the onscreen shuttlecraft. The exterior was off as well. It looked like he was basically meshing elements of the onscreen shuttlecraft with the AMT model kit that was out at the time.