ZZ has been around and well known by cubers for more than a decade, and for a long time had similar popularity levels to Roux since at the time, 3x3 hardware made M slices bad and RUL good
If ZZ were equal, the best CFOP solvers in the world wouldn't be more than 2 seconds faster on average than the best ZZ solvers we've seen (ZZ WR official average is 7.6x with EO cross variant, and I don't think we've seen anything faster than a mid 6 avg5 as far as unofficial results are concerned, which is again 2 seconds slower than the best CFOP averages)
I also think that the EOcross variant of ZZ isn't even really ZZ, since every step after EO is exactly the same as the CFOP steps with some advanced LL stuff (which we already see in CFOP) plus an F2L state that can be solved RUL gen (which isn't even optimal because of cases like R D' R' U R D R' and possibly setups to it), but this is a discussion for another time
If you or anyone else wants to prove that ZZ is on the same level as CFOP, then the actual core steps of the method need to fundamentally change, because EO>Line>RUL blockbuilding>ZBLL has obviously failed to compete with CFOP (though I realize that most ZZ users have figured this out and started pursuing EO+Cross)
EO>Cross>F2L>ZBLL also doesn't seem like it would be enough based on a few things
1st, obviously you want to do a mix of EO and Cross at the same time, but on a practical level they're going to be mostly separate steps the majority of the time, so I don't think it's a stretch to say that you're essentially making EO your first step for this method
(If any ZZ solvers want to prove me wrong on this and show me say, an average of 12 where you're consistently doing two or more cross edges before EO is done, then please do because if this assumption is wrong then everything else I'm about to write is at least partially wrong)
With that assumption in mind, you absolutely have to ask the question "is adding this additional step of solving EO worth it?"
The way you work that out is simple (in theory at least). You have to see how long it takes to solve EO time wise on average, and from there, see how much time you save with every other step (those steps being Cross, F2L, and LL)
Cross I would have to believe is worse to be honest, because #1 you're restricted to R,L,U,D,F2,B2 instead of R,L,U,D,F,B and #2 your capacity to plan a good Cross from a fingertrick/efficiency standpoint is never going to be as good as a CFOP solvers Cross
This isn't up for debate based on my original assumption... It's not a case of it being "harder", it's a case of it literally being worse, because you're trying to plan a Cross on a cube that's scrambled while tracking the 4-5 moves of your EO in your head on top of it (all while being restricted in what moves you can and can't do)
Trying to do CN ZZ only makes this problem worse... Technically on one EO axis you have 4 cross options if you do z/z'/z2 or no rotation after EO, but then you have to check all 4 after committing to an EO axis, and if you find a nice cross that requires a z2, you also have to ideally rotate and change your EO solution so that your solve start isn't "solve EO>z rotation>solve Cross"
If you want to say that hypothetically it's possible to plan EOCross+1 every solve, then I'm going to say that it's hypothetically possible to plan Cross+2 with CFOP every solve. Inspection wise, you're always going to be behind CFOP by at least one "step" (counting each F2L pair as a step) if it's truly a level playing field (I would argue more but I want to give y'all some breathing room)
Based on all of the reasonable theory-crafting I can do, I'm going to go ahead and say that EO first makes Cross a lot harder since once you commit to an EO axis, you're down to 4 possible Crosses instead of 6, you can't use F or B quarter turns, and if you're picking the nicest cross out of the 4, on average, you're going to have to adjust your EO solution 3/4 times if you want to be optimal and avoid doing a z/z'/z2 between steps
Good luck planning a first pair on top of that in 15 seconds (really more like 12 because everyone panic starts on the 8 or 12 warning)
With that now in mind, I think the next question to ask is "which of the two post-inspection scenarios is better?"
a) A Cross solved with all of your edges oriented for F2L/LL
b) A Cross solved with your first pair also solved
This wall of text is already getting very long, but I'm going to go ahead and assume that anyone who's gotten decently good with both CFOP and ZZ would agree that B is a lot nicer
People have already said a bunch about move-group stuff in this thread, but I'll go ahead and say that there's a reason most people don't use RULgen algs for PLL, even though virtually every adj/diag swap PLL has an RULgen alg that's the same movecount (or better) than the RUF/RUD equivilents (a good example is the R U2 R' U' R U2 L' U R' U' L Jperm)
It's because RUL is worse than RUF, RUD, and a variety of other move-groups. Again, RULgen PLL algs exist, and I'm fairly certain all of them are very close to the optimal FMC movecount, so if RUL was a good move-group, all of the fastest cubers would be using these algs since low movecount+good move-group is a complete no brainer
Finally, is ZBLL better than OLL>PLL? Probably, but not by more than half a second in my honest opinion
The only full ZBLL sets that I can safely say have been proven worthwhile are U, T, and L
Sune and Antisune have the worst CP recognition of the 7 sets, along with the fastest normal OLL's of the 7 sets. This makes ZBLL for these OLL's very hard to justify, even though on paper the algs are actually quite good (especially for Sune)
H and Pi have a unique issue where they have the easiest CP recognition (though only slightly better than U/T/L IMO), but also the worst ZBLL recognition due to the lack of any oriented corner
At least with Sune/Antisune, you can use a 1x2x2 oriented square to recognize things somewhat nicely, but with H/Pi you have to be a lot more creative with how you recognize the cases
I've talked with the best ZB users in the world, and even they agree that they only think parts of H/Pi are worth it for ZB, while some COLL sets aren't worth it
On the other hand, even though I don't have the best TPS or even turning, I've been able to get most PLL's sub 1 framecounted (can probably do the last two which are Fperm and one of the Nperms but I'm lazy), and almost every OLL sub .8 framecounted (minus an OLL where I do FRURUF>T ZBLL in actual solves), and with ROLL/JOLL recog tricks, I know that without learning anything new I can probably make my LL 2.5ish on average even if I stopped using ZBLL stuff/1LLL stuff/VLS stuff entirely
Still, I think all edges oriented last layer (aka a mix of ZB and OLL>PLL) when mastered would probably be low or sub 2, so this is the one area where I think EO as a first step could make a half second difference in theory
Bottom line, EO makes Cross worse in the sense that you're one step behind a CFOP solver inspection wise (if not more, I personally think it's closer to 2 than 1), but in theory makes LL half a second faster
Is RULgen 4 pairs is better than random-gen 3 pairs? Considering fast cubers can do a y2 in sub .4 framecounted (and y/y' a lot faster), along with the fact that good CFOP solvers won't rotate more than twice in a solve via rotationless F2L tricks like f R f' stuff, r U' R' U R U r', and a billion other neat tricks I could list, I highly highly doubt it
ZZ is a lot worse than Roux & CFOP, and that's OK