Given that this EZ is marketed for less experienced shooters, it would be reasonable to expect reports of failures. I could be mistaken, but the philosophy of the EZ was to ease the process of chambering a round, not a "no-recoil pistol".
I agree with the sentiment that a Shield will not resolve the matter anymore than giving an inexperienced driver a faster car makes that individual a better driver. 9mm is a "hotter" round and some may believe that will result in more positive extraction, however, pistols are engineered/tuned for their respective calibers.
Grip is CRITICAL and based on your report, the culprit to the stovepipes > break-in factor.
I recently purchased a pistol I had my wife shoot before me. She shoots occasionally, but still struggles with things second nature to more experienced shooters. She had a stovepipe every few rounds, which obviously concerned me. The ammo was Aguila, I consider it a weaker load, and it had been sitting for years, but still, every few rounds? I picked it up and put mag after mag through it with no issues. Lemon pistol? No. She has trouble filling the gap between the dove-tail and the top of the grip, this error allows the recoil to pivot at that point, and the business end will rise and twist, resulting in stovepipes.
Most shooters expect a break-in period of a few hundred rounds, especially on pistols they carry for self-defense, this allows components to wear/polish themselves in certain toleranced areas, springs to soften, and generally everything melds into the machine it is designed to be. In addition, any issues relating to reliability are likely to be experienced in this trial period. There are manufacturer defects, but all factors considered, the type of failure reported with a new pistol, being used by an assumedly less experienced shooter, and your inability as a shooter to replicate the failure-- it is operator error.
Do not overlook the importance of proper grip. One does not need forearms of steel to manage recoil, but proper grip, in every nuance, is critical.