Radio interference issues....
AM radios are more susceptible to interference caused by metal parts in Nitro rigs moving around (pistons going up and down, linkages moving, etc). You can really see it when you put your glow ignitor onto your glow plug before starting. My Losi RTR truck always twitched the servos when I attached the ignitor to my engine. Other than that, had no other issues...
The Spektrum-style radios using digital encoding are mostly immune from external interference caused by metal to metal induced noise...
The nice thing about running AM now is most everyone else is running Spektrum style radios at my local track, so almost all channels on AM are available when you get ready to race. Less chance of frequency conflicts nowadays...