One of the things that Traveller has suffered from in recent years is that when confronted by the sheer amount of material, many people’s brains just lock up and freeze. They seem incapable of understanding that, because Traveller is trying to support a very broad range of players, there is a lot of *optional* material.
For example, if you are setting a Traveller campaign on Earth then you don’t need anything that specifically relates to the Spinward Marches ... of the Gateway domain ... etc.
It’s the same or the T5 ‘core’ rules. Unless you are featuring clones or chimeras in your campaign you can ignore the rules on genetics. However, if at a later date you change your mind, it’s nice to know you have rules to cover that on hand. Other games would start with a ‘simpler’ set of rules and then address gaps with bolt-ons conceived as afterthoughts and with only partial success.
At its heart, the core of the core (so to speak), is an overhauled version of T4. So if a referee is already familiar with T4, then it shouldn’t take too long to become familiar with the essentials and be up and running a game. (Yes, there is some disorganisation but no more than a typical set of D&D.)
That’s not to say I’m happy with everything in T5: I prefer rolling high rather than low, I’ve always used the ‘advanced’ character generation careers rather than the ‘basic’ ones, and I want to see the big ships (cruisers and dreadnaughts). But there are more things in T5 that I like than I dislike. I had intended to treat T5 as a toolbox, mine it for ideas to add to my preferred rule set, but from what I’ve read I now intend my next campaign to be actually based on T5 instead.