Since I upgraded to Android 2.2, I noted some minor disadvantages of moving an app to SD card, like having to wait the SD card to be mounted after a reboot to be able launch the app.
What other disadvantages are there?
taken straight from android developer site
Also -> Applications That Should NOT Install on External Storage (more details on android developer site)
Applications installed on SD card cannot run if you remove your SD card or you're turning on USB Mass storage (so it can be accessed from a computer). That's the only drawback I have been experiencing.
I've actually seen performance improvement since moving to SD; before I moved to SD I had exhausted the internal memory (about a few kilobytes left) and the phone lags quite badly. Since moving to SD (and therefore freeing some space in the internal memory), and the lag disappears.