To me, the biggest drawback is its popularity: the hardware is insanely fragmented. Want to write a Symbian app? Browse the device list http://www.symbian.org/devices
App developers have to support:
1) mix of touch and non touch screens
2) Insanely different display resolutions
3) Crazy list of hardware buttons (some have keyboards, some none, some have the 10 digit numeric, etc.)
4) Different form factors (clamshell, block, etc.)
Basically, writing a very good, elegant app that people WANT TO PAY FOR in Symbian is a disaster. Best to write for iOS and Android. Although both hardware platforms are fragmented they are not nearly as bad to deal with as Symbian. That, and there's a culture of "It's OK and normal to buy apps" (much more so on iOS than Android, of course) that doesn't appear to exist on other platforms (yet).