A guarantee of one zetaeach challenge day would yield at least 3 a week and make that plus shipments a viable way to yield the zetas.
RNG seems appropriate for who the aat targets in raid or sometimes getting lucky and EP only stuns one toons. But really RNG after you have gotten through the difficulty of farming and building to even get to the challenges.
In fact, why in the world would you need RNG for zeta?! I understand stun cuffs cause it keeps people wanting to spend to refresh energy and shipments etc. but RNG for zeta only frustrates players.