The New Zealander won the first of the two Brooklyn races, following the red flagging of the race as torrential rain enveloped the circuit, which caught Cassidy in an aquaplane moment and sent him into the Turn 6 barrier.
Lucas di Grassi and Stoffel Vandoorne followed Cassidy into the wall, both hitting the stranded Envision car and causing significant damage - requiring a completely new car to be built up for the Kiwi to take to the second race.
Cassidy rewarded his mechanics for a late rebuild job with pole for race two, but was handed a 30-place grid penalty after taking a new battery and associated cooling components - sending him to the back of the grid.
As the full penalty could not be served, Cassidy had to complete it with a drive-through and thus ended any hopes of a recovery drive through the field - which left him feeling frustrated.
"I don't agree with the penalty," Cassidy told Autosport.
"If you change battery due to a fault of your own, you get a whatever penalty. I mean, I was in a wall, front on and I've been hit by two other cars through no fault of my own, damaged my battery and I've been penalised for it.
"And those two other cars, one's had a podium today - so that really sucks. And then to make it worse, my car today was unbelievable. I was so fast.
"So you don't always get those weekends in Formula E, the drivers here are really top quality and I think the order changes weekend to weekend because for the teams, sometimes some tracks suit you, sometimes they don't.
"And to have a car like I had today and not being able to start where we should have done, it's pretty frustrating."
Cassidy explained that he and his Envision team treated the race as normal, and wanted to compare his pace in free air to that of race winner Antonio Felix da Costa - whom he had beaten to pole.
He added that as a whole, he was proud of the weekend and the team's pace was encouraging for the upcoming London E-Prix.
"I still look at today because I'm super proud and I'm super happy of the weekend," Cassidy reflected.
"To do two poles the way we did, the first race the way we managed it I think before the rain hit we were we were home easy anyway. And then our pace and today's race was unbelievable.
"We treated it seriously; we didn't try to save energy, we wanted to see what our pace was versus da Costa because he was in clean air and I was in clean air.
"And I had unbelievable speed in today's race, so it's really nice to see, to show, I guess. It gives everyone a bit of confidence for London as well."
