Other than portability, are there any other advantages (or disadvantages) to storing apps on external storage?
taken straight from android developer site


There is no effect on the application performance so long as the external storage is mounted on the device.

The .apk file is saved on the external storage, but all private user data, databases, optimized .dex files, and extracted native code are saved on the internal device memory.
The unique container in which your application is stored is encrypted with a randomly generated key that can be decrypted only by the device that originally installed it. Thus, an application installed on an SD card works for only one device.
The user can move your application to the internal storage through the system settings.



Also -> Applications That Should NOT Install on External Storage (more details on android developer site)


Services
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Live Wallpapers
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App Widgets
Account Managers
Sync Adapters
Device Administrators
Broadcast Receivers listening for "boot completed"