From an entertainment industry / ancillary market standpoint, FJ's work was revolutionary (like many TOS market releases). Today, sci-fi schematics are commonplace, with many thinking Star Wars was the production to make that a popular piece of merchandising, but FJ's work on TOS tech broke ground on so many levels, it is little wonder the Technical Manual and fold out blueprints were runaway bestsellers in the mid 1970s.
At the time, I too realized FJ's Phaser, Tricorder, Enterprise pylons and other details were not screen accurate, but it was such a massive look into the world only hinted at in The Making of Star Trek (1701 & Klingon illustrations), that accuracy took a back seat to the joy of seeing an expanded TOS Starfleet.
I still rate it as one of the most important, must-have books for TOS fans and historians.
I heartily agree with all the praise. Regarding what was inaccurate, you can add the size, shape, and weight of the communicator. But what was wrong with his pylons?