From an email with Jim Lewis.......
The actual issue is related to how typical X2 Capacitors lose capacitance over time. A typical film capacitor loses capacitance, over time, because of two effects: 1) corona and 2) corrosion. The voltage applied to most X2 capacitors tends to be in the 120-277 range. Corona occurs around 300 Vac and higher. So, while some capacitance might be lost because of corona effects, this isn’t the main concern. The real concern is that moisture will eventually egress into the package and oxidize the metallization. Some plastics, like polyester, more readily absorb moisture and accelerate this effect.
In typical X2 applications where the capacitor is across-the-line, the capacitance loss isn’t a concern. First, the capacitance value isn’t critical and second, most people don’t care if the end-product passes EMI tests 4-5 years into its life.
When used in a series-coupling or series supply application, however, the loss can be critical. Even a 10% loss in capacitance can result in not enough current for the supply.
This comes down to (and this applies across the industry), most X2s were not designed with “stable capacitance” in mind. So the R46, our most popular, can easily lose up to 30% of capacitance in a high humidity environment after only a couple of years. Which is why we made series with “better” epoxies, thicker metallization, and series-electrode constructions. Their capacitance loss will be much less than a typical X2, making them more suitable.