I backed Marc Miller's Kickstarter campaign for Traveller5 a number of years ago. The backer rewards arrived years ago as well, but I never quite looked at the book in any sort of depth. It is a bit intimidating, being bigger than a copy of HERO System Fifth Edition Revised. I finally looked at it in depth four years after receiving it.
Let me preface this review by saying I have not sat down and tried to play a game of Traveller5. There is indeed a game in here somewhere, but the bulk of the book is not dedicated to a game.
It is actually a giant toolkit that you can use to build things for other editions of Traveller. That's certainly why I was reading it last night, and definitely not for the T5 game itself.
There are some serious problems using T5 even for that, however. The biggest barrier to using T5 is the fact that there is no index for a 656 page rulebook. If the book had an index in addition to the table of contents, the second biggest problem would be no problem; there is a lot of game-specific jargon floating around. An index would allow me to just go look up what QRBS is an acronym for, or find what the heck a Hop and Skip drive is when I encounter it perusing the Technology chapter. I never found what a Vault Drive is, but I presume it's a FTL drive that jumps thousands of parsecs given that the other drives are powers of ten compared to the humble Jump Drive.
The other problem is that Miller digresses into little essays about alternatives. While this is really interesting stuff to consider when world/universe building, these alternative thought experiments really could have been collected into a collection of essays in some appendices. The philosophical discussion at the beginning of the Technology section about what the definition of technology is... is very fascinating food for thought in the context of alternatives to technology within a game universe. However I feel that it does not need to preface a chapter on the game rules concerning technology.
I feel that the third biggest hurdle is the density of the text itself. There are a lot of formulas and algorithms in the book that detail out how to generate and describe stuff. Very useful due to the vast expansive nature of any Traveller campaign, but those things obfuscate things just enough that they're not useable on the fly like much of Mongoose Traveller's generators and descriptors. The details provided are extremely useful; if you plan to translate it to a different edition of Traveller, none of which are as detailed as T5.
Is it all bad, I hear you wondering? No, not at all. There is a lot of detail to be had in the generators the system provides, and the core philosophy I saw running through the book is that you should only generate or describe necessary details. There's an acronym in the Stars and Worlds chapter: MOARN. Map Only As Really Necessary. Because most of these generators allow you to stop and start as needed, you only need to provide necessary details rather than all the details. The ThingMaker and GunMaker sections are incredibly useful for determining characteristics for new stuff that isn't a starship, something lacking in most editions of Traveller I've read.
Hilariously, there's something called an N-Drive in T5. It's short for NAFAL Drive, which in turn means "Not As Fast As Light". It's an alternative for more traditional Traveller Jump Drives, and as the name implies, not nearly as fast- An N-Drive covers a tenth of a parsec in a week. Great for in-system travel, not so great for interstellar travel.
I haven't even talked about the dice mechanic. In essence you're using six-sided dice in varying amounts, trying to roll under a target number. The more dice you roll, the more difficult something actually is. Miller devotes a whole chapter to the probabilities of this core mechanic, which is useful for a GM trying to understand how to determine how difficult a roll ought to be. But it is not as useful to players who care about just knowing how to roll and read the dice. This style of die mechanic is not one I prefer, but it's a solid one. It's far more sane than how Big Eyes, Small Mouth Second Edition handled things, which quickly got out of control.
Overall I think the problems with T5 are primarily organizational. An index and glossary would go a long way towards making the rules comprehensible and accessible. Beyond that, separating the generation rules from the game play rules would also help. That is, take the ThingMaker, GunMaker, ShipMaker, etc... and put that into its own volume. The stuff about how to actually use the products of those maker rules needs to be its own thing that can be brought to the table in something smaller than a ~650 page hardback.
The long and short of this review is this; Traveller5, in its present form, is better used as a toolkit for other games and not as a game itself.