HF compression drivers are typically worse than domes with respect to air nonlinearity. The problem is twofold:
- Compression ratios are usually around 10:1, meaning the total area of the phase plug openings is 1/10th the effective diaphragm area. For a typical 1" exit driver with a 1.75" (44mm) voice coil, this means that the sound is "squeezed" through an area similar to that of a 0.55" (14mm) dome. SPL is thus extremely high near the phase plug entry.
- The area expansion is usually much slower, which further increases distortion (2nd harmonic dominant) at high frequencies. In this case, the mechanism is a mismatch in propagation speed between the high and low pressure regions of the sound wave. The pressure peaks "pile-up" against the troughs, gradually distorting the wave toward a sawtooth shape. Higher frequencies are distorted more than lower frequencies and the effect is cumulative with propagation distance—long, slow-expansion horns have higher distortion at HF than short horns for the same throat SPL.
Look at harmonic distortion measurements of some good loudspeakers with HF compression drivers (examples: Genelec S360, JBL 4367 and M2) and you'll notice rather high 2nd harmonic compared to good dome tweeters at the same SPL. Also note that the 2nd harmonic increases with increasing frequency due to #2.
Personally, I'm not so sure the above effects are of much audible consequence at normal domestic listening levels. My own speakers have compression drivers and the sound is top notch (in my opinion, of course).