This RNG PvP strongbox system is going to cause a lot of problems.
How great it is going to feel getting 3 mythic bracers in a row over 3 weeks?
While someone else is going to get a chestpiece, a helmet and leggings.
Now, lets say 4 months into the PvP season: A lucky PvP player is going to have full or almost full raid gear and an unlucky PvP player still has dungeon blues in half of his gear slots.
This kind of system is going to get a lot of people trying out PvP but I don't think many are going to like it. In fact, many are going to end up hating it after spending time getting rating and then getting rewarded with something useless.
As a PvE and PvP player my luck with PvP strongboxes is going to determine my performance in raids and the new challenge mode dungeons, not great.