The big issue with ANT+ is that it requires data every second, on the second. If crank-based systems are measuring rate of pedal rotation with a magnet, they get a signal only once per pedal rotation, which is not going to be exactly every second.
Is this really so big of a problem? Your pedal revolution is almost always going to be faster than 60rpm, so for every ANT+ update you'll have a new estimate of the cadence. In addition, it's not like cadence varies wildly over a span of 1 second, so even if you used the cadence value from one second ago, it's not like it's going to be off by 30rpm. Also, every other crank-based power meter has the same problem and have found a way around it, so it can't be that big of an issue. And then finally, you typically average power measurements over a span of 30 seconds anyway, so any intermediate power errors get averaged out anyway.
Yes, making a power meter is hard, but let's not pretend it's rocket science. Otherwise the dudes at Stage One wouldn't be able to glue on a 20g plastic device and have it work just as well as the other major players. I think the biggest issues are just calibration and testing as you're using strain gauges on a certain crank arm and assuming some properties of each crank arm based on a set of collected data.
There's two issues: (1) sampling cadence only once per second, (2) the time lag of cadence relative to torque.
On (1), I looked at that here.
I concluded for low-inertia pedaling (up a steep hill, for example) the error would exceed the claimed 2% accuracy, steady state and not just transient.
Power2Max doesn't suffer from this issue since it uses accelerometers. It may suffer from other issues...
So I don't think SRM and Quarq are accurate when the pedal moves at a nonuniform rate.