I wasn't actually specifically indexing rules light as the key measure of complexity, although most people in the thread do seem to be doing so. I mentioned BW more because of the cognitive load of meshing the various systems (the spokes and rim to use their terms) into functional gameplay, on both sides of the screen really. BW certainly can and does collapse back to basic principles in a moment of doubt, much in the same way as PbtA games do, but it is also a game that explicitly references system mastery as a goal of play and a game that, perhaps more than any other I'm aware of, rewards increasing system mastery with increasingly positive play experiences. Anyway, that's my two cents.
The cognitive load of BWR/BWG is certainly heavy... but is it complex?
I argue not - because it's process based, and all the various options retain the same process. Conflicts beign a singular table of action interactions, and the opposing actions being rolled based upon the fiction state (for what skill) and the chosen actions (yours and your opponents) determining who rolls, and who is elligible to do damage (wounds or disposition).
The concept of disposition is not terribly rough, either: it's a damage track for non-combat based upon a stat and the roll of a skill suitable for the situation.
In terms of Mechanical Complexity, for BWR, the tables I needed:
Fight Actions crossreference (1p)
Duel of Wits actions crossreference (1p)
Range and Cover actions crossreference (1/2p)
Weapons Tables (1/2 p)
Skill Advancement table (1/4 p)
List of standard modifiers to a task (1/4 p)
List of talents and their effects. (1p)
That runs to abut 7pp. Adding the skill list and a few other useful bits pushed it to 9pp
For comparison, to run Dune 2d20, I'm up to 7p, including playtest material and all of char gen!
For T2K 4e, my "routine uses" are 4pp:
Crits (1p)
Small Arms Weapons list (1p)
Direct Fire small arms process (1/3p)
Actions list (5/9 p)
Melee process (1/9p)
Travel and Encounter process (2/6p)
Healing (1/6p)
Encounter types card list (1/2p) (only because I'm using standard cards, not the custom deck)
But the tables overall, excluding weapons lists and vehicles list, are some 14 pages.
I routinely need one or another table that's not in the "common use four pages" above, such as artillery procedure, mines, scavenging, etc... The scavenging rules alone take more than a page (1p is just the table of things found by searching. One roll per success).
Now, from a player perspective, BW has the players engage with the combat tables pretty directly. T2K doesn't. 2d20 has some engagement, but the players tables amount to a 3x5 card's worth.
In play, all three seem to me to be comparable complexity, but the objective measure (how much must I look up to run) puts BW well under either Dune or T2K 4E...
But, comparing what the players need...
In BW, all the tables I need for running save the traits are ones the players are expected to interact with, directly or indirectly; it's easiest to simply let them have them.
In Dune, the only ones needed are "how to build the dice pool" and "how to spend momentum"... all actions boil down to "move self, Move asset, Attack using asset, create an asset"
In T2K4, players need the actions lists (SA/FA), and the terrain effects. that fits nicely on one side of a page. Recovery rates can go on same page. It helps to have the direct fire and melee resolution processes, but they don't need it, as it's technically a GM action.
In T2K4, the mechanical weight is on the GM. Haivng the right tables either memorized or to hand speeds play.
In Dune, it's on the GM and players about equally.
In BWR, there is a shift of slightly more over to players than in the others listed... but, at the same time, BWR has a bunch of "shove this off on your players if you want the game to be playable" - calculating experience checks being the most obvious, and amonst the most tedious, such chore.
Dune hides its complexity - it's got a lot of non-called-out interactions... such as when is it better to attack the asset or its wielder? Is the action one of using the asset yourself as a wielder, or as a remote element reflecing your influence outside your presence? What does Trait X do at this moment? (That's one of 5 answers: Allows a normally impossible action, disallows a normally possible one, provide a -1 difficulty bonus, or a +1 difficulty penalty, or the bland "Nothing"...) It's complexity is that the GM and players are always negotiating the value of their assets in the current fiction. And with a much weaker mechanical state than BW, that means a lot more "playing the GM" than "playing the game"