it uses AM. This is one of the most interference prone transmission medium in use at the moment.
Yes it is suscetible to amplitude based interference such as ignition noise and vacumn cleaners but how often do you hear those? AM especially at VHF, where you avoid most of the propagation problems that cause interference, is quite capable of providing good quality audio. Heterodyne interference is probably the only one that blocks reception but is largely eliminated by good operating practice. I totally agree that most light aircraft radio installations are of very poor quality but it is mainly associated with poor engineering and nothing to do with the mode of transmission. In most cases the culprit is poor matching causing boxes to overload and go into distortion. Then add in incorrectly set Squelch levels.
The secret of listening to RT is to know the contents of the various messages so that you only have to fill in the bits of variable information.