This thread is crazy. Having actually played the game, I think the balance of gameplay styles is better in The Lost Legacy than in A Thief's End.
It all depends what you consider to be "gameplay". If the only kind of "gameplay" for you involves shooting people in the face, then the combat is used sparingly, but there are some good encounters IMO.
But it's obviously not the only kind of "gameplay" to begin with. The Lost Legacy has quite a lot of puzzles, both environmental (I can see where I need to be, but how do I get there?) and more traditional ones that you need to complete to progress. This is all mixed with the exploration (in the series' largest ever level no less) and platforming aspects.
Like I say, I think the balance is right here. It feels like an Uncharted: Greatest Hits to me, which I find very agreeable personally.
I'm glad Naughty Dog mixes it up with different gameplay elements, rather than pit you against waves and waves and waves of soldiers like the earlier games did. I feel like the series has matured in the right way.
I haven't played LL yet, but I've played all the console Uncharted entries.
There's no question everything the player does while engaging in a game counts as gameplay—from light adventure games like Gone Home/Firewatch, to Bayonetta/DMC, and everything in between. Even low-impact gameplay such as choosing dialogue options or inspecting environments/objects can be engaging.
The hangup with Uncharted's non-combat gameplay is that it has always been merely complimentary to the combat. The climbing, the puzzles, and now the exploratory segments are not fully realized like the combat scenarios.
About the climbing:
I've played Grow Home and Grow Up, two games entirely about climbing. Putting aside how long they are (they can be beaten in an hour or two, or can be 100%'d which could take far longer), the moment to moment climbing is engaging because the player must hold a trigger/key in order to grip onto a surface. There's no stamina meter, but if the player lets go of a surface with the robot character's left hand before attaching the right hand to something, they'll fall, and unless they catch themselves, they could lose progress. What the player does in moment-to-moment gameplay matters.
In Uncharted, The climbing itself requires no timing or inputs beyond pushing in a direction and pressing X. In a static environment, it's often impossible for the player character to fall due to player error—when there's a bunch of crazy stuff going on, that's a godsend as the player's attention is split between leaping between/hanging off the sides of trucks and fighting enemies, or hopping from roof to roof while dodging helicopter fire. Even during the UC4 auction stealth segment, the streamlining of the climbing only helped make for interesting scenarios involving pull guys off ledges or avoiding their flashlights. The traversal serves the action well, but it does not feel complete on its own. Stuff like swinging around with the rope, sliding and the piton are great because they introduce the need for actual timing and planning, but they're underutilized and hardly are ever mixed-and-matched with one another (until the penultimate chapter), and sliding rarely gets mixed with combat outside of the Scotland ending and that one arena that was shown at E3 '14. There's nary a moment where the path forward requires any discovery or thought, so merely going through the motions can get really dull when it's this frequent.
Exploratory segments:
These areas in UC4 are gorgeous, and the island hopping one and the Madagascar areas allow the player some agency as to where they want to go at certain points, but exploration is rewarded with some random artifacts that have no contribution to the story or the gameplay—you can't sell those for money to get more weapons or tools, and Drake doesn't describe the history behind the treasures (which he never did in previous games, but those were always just slightly off the beaten path, whereas in 4, there's stuff like the actually neat Madagascar well puzzle that leads to ... a random piece of treasure). Finding the diary entries near old pirate corpses is cool because they parallel the main story, but that's pretty much it. Any hidden weapons will have just a few shots, and there's no real feedback loop for exploration—you just do it, maybe find a treasure or power weapon (with no way to get extra ammo) and then it's back to the main path. Stuff like the painting room was entirely a series of looking through empty drawers until a cutscene triggered.
Contrast that with something like Firewatch where the dialogue choices affect further dialogue and give some insight into the characters involved. Uncharted 4 has maybe three moments where the player can choose dialogue, and beyond that it's all pretty vapid banter. The only times the conversation felt substantial was during the island hopping part with Sam as that really showed how bummed Drake was at that point, and the poriton with Elena. Finding objects in Firewatch often had some story purpose, and picking stuff up is a core game mechanic, and there's plenty of stuff to find in Firewatch, even that stuff doesn't loop back into — it's a way to interact with the game world. There's really no interacting with the environments in Uncharted because every input on the controller is for combat purposes—there's no dedicated "pick up/inspect object" button, but there's a cover button, punch button, aim trigger, shoot trigger, reload button, etc. The exploratory segments feel like afterthoughts, and play as if they're complimentary, yet they take up a lot of time, just like the climbing segments.
That's why I don't consider "balance" to be giving each of these types of gameplay anywhere close to the same amount of run time. If the climbing itself is super basic, it doesn't need to show up for long stretches of time. Following linear paths up cliff walls without any real choice of where to go and without any need to consider my inputs or time things well does not hold my interest. Walking around beautiful environments losing its luster when there's nothing for me to do or find that has much significance to the story, the lore or the gameplay. Merely hearing scripted quip-filled conversations between characters in which I have no influence over that don't tell me anything I didn't know about the characters already doesn't make the dull, lengthy climbing segments and empty exploration any better.
I would love a game that incorporates everything Uncharted 4 attempted, being equal parts traversal with exploration and combat in a treasure-hunting setting, but it would all need to be fully realized and not just feature the most basic, simple versions of each of those types of gameplay. It's why hearing that Lost Legacy has even more of surface-level exploration and insubstantial downtime is disappointing to many.