Okay, I may have missed it and can't find it under search but...I don't know what the drop rate is for zetas on the challenges Let's just say the drop rate is 50%. Which I would consider high for E...
For what it takes to get the ships you need and then 3-star the challenge, you should at least be guaranteed 1 zeta with some x% chance for 1 or 2 more. The fact you can 0 zeta after two challenges is garbage. Imagine gearing up a Scoundrel squad to have a legit chance of getting 0 credits after two Credit Heist attempts? Same difference...