It’s easy enough to verify whether or not a product is certified. The more important question is, has the product been tested at the advertised flow rate by a third-party, ASSE-listed laboratory—and did it pass?
It’s a common issue, especially with older residential buildings. A toilet flushes and the water pressure in the sink drops, or the shower turns scalding for a few seconds before stabilizing. In a single-family home, it’s an inconvenience, but in a multi-family home, apartment building, or other commercial application, it’s a major problem.