The masking problem stems more from a misunderstanding than anything else. The difference is in how the units behave under interference. PCM rejects bad frames, this results in hesitation or jerky response that is less obvious when compared to ppm units that twitch all over the place trying to move servos to bad position information. What is often not discussed is the fact that this hesitation will not even be noticeable until interference is large (amplitude and duration) to render the PPM unit unflyable. You'll often read of pilots preferring to "fly though" interference with PPM than be "locked out" of control

This PCM 'lockout' myth is another common misunderstanding. Failsafe is engaged sufficient back-to-back bad frames (frames dropped due to being scrambled by interference) are received to enter failsafe (varies by manufacturer but, typically ~30ms). This mythical 'lockout' is the radio entering failsafe mode.... You literally have lost all potential for control due to interference. Control is returned when a number of good frames (again, varies by manufacturer but is typically less than 10ms) are received.

It's also significant to note that the fact that PCM has a better ability to reject noise.
In fact, the signal to noise ratio tolerance is so significant that PCM units will continue to operate under conditions that would render a ppm (often misreferred to as FM) system useless. In other words, you would experience complete loss of effective control with a PPM unit before any noticeable impact on a PCM system. This is why many people suggest switching to PCM in conditions where interference is too great for PPM units (e.g. Gas/ignition engines). This is not good practice as interference has a tendency to increase over time. You might get by for a while but, eventually you'll get bit.

HTH, Jim.