If you want it to operate based on power zones, you need a AXS-compatible power meter, but I believe you can use it with the Favero pedals and an AXS spindle if you don’t want it to use power zones.
You can’t use FA with Favero pedals, only Sram powermeter is supported
SRAM has a fairly low cost cadence meter that allows you to use FA without a powermeter. The cadence sensor only works with certain SRAM cranks, but I think that is most of the current stuff since transmission was launched. If you want FA to use power data, it has to come from a quarq/sram PM.
In my experience, FA works reasonably well without the power zones set up, but better when they are set (and you have PM data). That said, my experience riding without the power zones is pretty limited. When you start up a FA system for the first time, it takes some ride time before it determines your power zones (unless you manually seeded them). So, it seems that the initial ride/setup time is only using the cadence data from the PM, basically mimicking a system that only has a cadence sensor. It’s still smart enough to unlock suspension when not pedaling, etc., but its not going to progressively firm things up based on how much power you are putting out. I choose to set my zones manually, you can alter system behavior significantly by changing the power zone thresholds. It’s a fun thing to play with if you like messing with that kind of stuff. I just look at it like any other suspension setting. I honestly can’t say it makes me any faster, but I still enjoy tinkering with it to make the bike behave to my preferences. Another reason I set my power zones manually is that I don’t want the system resetting stuff on it’s own based on ride data that might not reflect my racing. I can’t find any documentation on how often it adjusts, but I figure it’s better to just set it to what I like.
I have the flight attendant pedal sensor on my X01 mechanical cranks. My only issue is if you are out of the saddle it won’t lock out if there is bob in the suspension (even if on a flat, smooth surface). Moving from Pedal to Open (and back) is great. A manual lockout would be great but outside of FA my MTB is mechanical.
I give it a little more thought…maybe I’ll give FA a shot…and then realize that the $12k Epic 9 Pro has a cheap left side only power meter 
. So it would be another $1000 or so for a real power meter
unless you have a prior injury in one leg a one sided pm is sufficient. if you’re a seasoned cyclist your balance is probably within 47-53 or “better”. No need to spend more on a dual sided PM imho.
Electronic shifting is just better though. I only recently went to Di2 on the mtb (and 18 months beforehand on the road) and there’s no way in the world I’d go back to mechanical. But if you never try it you’ll never know what you’re missing 
(And I’m old enough to have used Suntour Superbe Pro!)
Agree,
I have dual sided power meter on my road bike and single-sided assioma on my epic evo 8.
I have 51/49 power distribution (left/right) according to dual sided power meter, so to keep both aligned, I just set correction coefficitent in assioma app to account a bit over-estimated power taken from my left pedal.
You probably missed my earlier post where I mentioned I have had electronic shifting and have it on a current bike. In that post I mentioned it works great, but also why I don’t like it. Anyway, it’s tough/impossible to find high-end bikes without it anymore and that even have provisions for running cables.
TBH I’m not happy with Di2 on the mtb. I wish in retrospect I took it off when I got the bike and sold it and put on mechanical. It’s fine but it adds nothing extra, is a little annoying to adjust, it’s heavy (although IMO 12 speed drivetrains are excessive and adding unnecessary weight at least for where I am), and now I have to worry about a >$500 part that is getting rocks thrown at it.
Technically correct. But practically relevant?
Yes. Why would I want to introduce 6% more variance in my data when I can just run a two sided power meter?
Weighted left side power percentage is 48.9006%. So you’re introtducing about 2.1994% of error which is probably within tolerance of most powermeters. Is it really important to do an interval at exactly 297W or is 303W okay too? It doesn’t matter to me. Left sided is good enough for most people who don’t have crazy imbalances.
Once again, the point isn’t that there’s an imbalance. I agree if it was always 48.9/51.1 it wouldn’t matter. The point is that the imbalance varies, for me at least from 47/53 to 50/50 just picking one ride from this week.
I could be mistaken but I think the point was that your body can’t discriminate a few watts and so it doesn’t matter. 
I do think that for me in MTB a left sided power meter underestimates a lot because I’m right foot dominant so on punchy terrain the right side does a lot more work.
Doesn’t matter too much but I’m glad I have a spider meter instead of a left side now.
I do wish that all high end bikes came with good power meters. I’d rather have that before electronic drivetrain and shock. Sucks to buy a nice bike and then have to play the power meter compatibility game.
Anyway.
Does the Epic 9 come with a mudflap at the lower linkage next to the BB. I always get dirt in there on my E8 and I’ve seen some hacks to cover that gap but got wide eyed stares the last time I visited a Specialized dealer and asked them about that.
My L/R balance changes depending on intensity and whether I’m out of the saddle or not. I’m right footed/handed. Below Z3, I’m typically 52/48. Then as intensity moves towards about 110%-120% of FTP, the balance gets to an extreme of 45/55 at times but usually about 47/53 or 46/54. Above that intensity, it moves back towards 50/50 or 49/51. If I’m out of the saddle, it’s almost always close to 50/50. That’s on a road bike. I don’t have power on my MTB right now. I imagine it’s even wilder.
Hot take… power meters don’t matter in MTB.
(Like real MTB, not Leadville stuff.)