FYI in NZ they are also coming down hard on singularly reinforced walls, basically previously you could use the standard 0.85 strength reduction factor, now you need to use 0.7. You are also not allowed to use singularly reinforced walls where walls are face loaded and part of the lateral system (as there is no confined concrete). Lots of other requirements/restrictions as a result of findings from the Royal Commission into poor performance in these systems in the Christchurch earthquakes.
Part of the reason is that concrete is much stronger over time than the design value, so rather than forming distributed cracking to dissipate energy you get a single crack (with lower reinforcement ratios at least) and hence reinforced capacity can actually be lower than the uncracked concrete capacity. This generally results in bar fracture due to low cycle fatigue after a low number of cycles, as might occur in a seismic event.
As I believe it the intent of the recently updated NZ provisions are essentially making sure that the walls behave almost in an elastic manner up to the Maximum Credible Earthquake (MCE, 2500 yr return period event).
There might be laboratory testing to show they are robust, but real events in NZ recently generally showed in certain circumstances very poor performance.
