Polypropylene and Polyester film capacitors fail just like any other type of capacitor, it's just the mechanism in which they fail makes them hard to determine if they've failed.
They're highly intolerant of voltage spikes, or being run over their rated voltage.
Generally, if a polyester cap is not exposed to over voltage (spikes or otherwise in excess of their ratings) it is extremely reliable over very long time frames. On the other hand, capacitors like Rifas, that use a metalized paper film are not. The voltage spike events corrode away the film and their uF value drops with time. Many X2 capacitors suffer this fate. That is why it is folly to use them in circuitry as AC voltage dropping devices, but many companies do it for small appliances. It is also a bad idea to use them in general circuitry for AC coupling or bypass applications. Many have problems such as cracked casings and water absorption too.
Ignoring the fire retardent properties of the X2 type, a polyester cap is far more reliable in the application, provided the voltage rating is high enough. As an experiment , now running a decade, I replaced X2's in a mains voltage dropping application (that lost capacity after 6 months with X2's) with 1500V DC rated polyester types, they are still fine becasuse the mains voltage plus the spikes,doesn't exceed their voltage rating, but it would for example with a direct lightening strike.In early tube gear wax paper caps were used as X2's, they actually seldom gave trouble, because generally they were 1500V DC rated, so were ok on line voltages.
The ideal capacitor though, to replace an X2, is the type used as tuning capacitors in horizontal deflection stages in color sets. These generally have a 1200 to 1500V DC rating and are fine on 240 or 120V mains and they have especially low resistance bonds between the plates and the lead in wires and extremely low ESR. But these, like polyester caps, are not officially rated for any spike suppression activities across the line power even though they would be more reliable than a standard X2. This is because of the fire rating and the fact that they can short circuit, but they should always be fuse protected anyway, and we have all seen Rifa's burn up.
In a coupling application, in a monitor where the peak currents are high, like a deflection yoke coupling capacitor, a polyester cap is better than a bipolar electrolytic and an oil filled capacitor is equally good (but bigger and more expensive). In other circuits where capacitors are involved with substantial circuit currents, such as motor run types, once they were all oil types, now , after going to film types, the industry has noted they degrade and also lose capacity over time in some cases, just like the X2's. But most likely, if they were physical bigger and had thicker insulation with higher voltage ratings they would be ok as my experiments with 1500V rated capacitors & mains power voltages have taught me. The DC ratings of X2 caps are normally lower than 800V and in my opinion not high enough, but everybody wants small physical sized capacitors.
