Initial Zenith products, specifically the roller delayed weapons, had some issues. Long story that had to do with the MKE side, but those have been corrected and the products being offered are very good quality. They actually went back to MKE and had them go back to the original specs from HK and start over, removing all of the "modifications" that had been implemented over the years and created the reliability issues.
I've been working with the company over the last couple of months and am currently "shaking out" a demo weapon they provided me. I've been to their current facility met, their staff, including their outstanding Armorer and customer service personnel, and seen their new facility that is about to open and increase their capabilities significantly. They've also got new plans, focused on the roller delayed weapons, that will be worth keeping your eyes open for. Great "family-centric" company dedicated to providing quality products to the US market, at competitive pricing.
Below is a link to an after action report I posted following one of our Advanced Sub-Machinegun Operator Courses last month. I invited my friend, Chase, the Director of Marketing for Zenith, to attend and he ran his Z5RS like a boss all weekend. The only issues I noticed during that time were a few failures to feed that we determined to be the result of one of the magazines, which had feed lips that had widened to the point of being out of spec. We attributed that to be due to mag drops during reload drills.