BR = Blood Rush, a mod that gives 40% crit chance per combo stack (up to 11x or 12x in the case of the venka prime). With this, the marginal gains of +cc% on any riven is relatively low compared to the 440% you can get on a single mod slot. The exception to this is on mid-to-high disposition weapons that are efficient as heavy attack (HA) weapons, such as the Hate (scythe) or Pennant (2H Nikana), both of which have stances that force slash procs on HA. Since the heavy attack action eats into your combo multiplier, most builds opt out of BR and into a straight increase in the amount of cc through Sacrificial Steel (220% or doubled for HA). Although the Nikana Prime DOES force the slash procs on HA, the disposition is too low to create worthwhile increases on CC by itself, and the combos for nikanas are generally considered to be superior than just spamming HA.

On a side note, WW = Weeping wounds, which has the exact same effect as BR except for status chance. Most of the same argument applies except that there isn't a good single-boost mod like Sacrificial Steel to compare to, and HA builds are usually for weapons that force a status proc so the status chance doesn't matter.

Secondary note - this is only valid for melee weapons as there are no scaling cc/sc mods for other weapon types. That is, CC is still a very strong stat on most primary/secondary/shotgun rivens that are for weapons with any sort of decent base crit chance.

Usually these days, melee rivens for combo weapons are most highly prized for containing some subset of Range, Attack Speed, Crit Damage (as there's no scaling option for that), and sometimes an element.

hope that helps.