I love Fire, Fusion, & Steel -- I know it is an unpopular opinion, but having in-depth 'making' rules is an important part of building a believable universe. For folks who don't care; just don't bother -- there are plenty of weapons, ships, vehicles, armors, robots and other gadgets in the core and 'world' books.
But getting into the nitty-gritty allows gaming at other levels; Traveller also spun off Striker & Striker 2 for platoon to battalion sized conflicts; Snapshot, Azhanti High Lightning, & At Close Quarters for person to person & squad sized fights; Trillion Credit Squadron, Brilliant Lances, and Power Projection: Fleet for clashes between ships & fleets of ships; Imperium for theater-level interstellar wars, and so on.
T4 'Pocket Empires' is highly recommended, -- it allows players to play the decades- or centuries- long growth and conflict between interstellar empires. But it fell down in the transition between 'Now my character has gotten to be famous, important, and wealthy! Time to establish a dynasty to carry my legacy forward!' at the late stages of a long RPG campaign and actually carrying that character & their accomplishments forward into the empire-builder game.
What exactly is a 'Resource Unit'? How do you translate a 'Plankwell' -- or a player's favorite custom ship design - over? A mercenary, with an elite and lovingly built division of combined arms troops? How do the various designs of stuff that the players build perform in 'the big leagues'? That sector-wide trade line that a group of Traders started from nothing?
The key is to have an underlying system for how things work, where stuff can carry over -- from the smallest to largest scales -- in sensible ways. Mongoose has failed to do this so far; they have hand-waved some stuff for rule-of-cool or maybe because it seemed easier at the time. But this sort of approach leads to discrepancies -- like Strephon mysteriously not having a Personal Shield Generator when Dulinor shoots him with a small caliber handgun. And putting off creating a 'build system' makes it ever harder as time goes by -- more ships that could never work with the current rules; more vehicles with bizarre prices, capabilities, and limitations, more weapons which simply cannot exist within the rules. Don't get me wrong, other games -- and other editions of Traveller -- have also failed to get this right; so Mongoose is not alone. The difference is that Mongoose could still excel above all those others.
Even if the 'build system' is never published (and I strongly believe it should be) it should, at least, exist.
Publishing the build system allows people to build their own toys -- and that becomes a rich resource for others to mine for new encounters, adventures, and so on. So I recommend, warts and all, T4's 'Fire, Fusion, & Steel'.
I do not have good spreadsheets and am crap at making them. 
IMO.
