At 3.8-3.9GHz there really aren't any gains. Power phases are used Alot for stability purposes, the VRM's convert 12v DC to @0.2-1.5v DC, but that 12v is subject to its own noise, it's never pure. Power phases help clean the resultant ups and downs even further, supposedly, but can introduce dips between phases, so more is not often better for that purpose.
More phases is only a benefit under extreme OC, where high current and voltages are split between more VRM's, lowering overall heat output. A 3+3 will over tax the VRM's long before a 16+4 will, but only if pushed hard in the first place. Most ppl are opting for the B boards, and have done so since Ryzen first arrived as they don't have the headroom Intel cpu's do, so don't require all the bells and whistles.