I have Windows 10 mobile, and I have never done a factory reset since upgrading. I have a large number of apps installed on my SD card. If I do a hard reset, will the apps on the SD card and their data get removed?
I'm not sure about resetting a phone using the hardware buttons during boot, but when you initiate a reset from the Settings → System → About menu, you can choose whether or not to also erase the SD card (by default that option is unchecked). If you want to be 100% sure, though, just remove the SD card from your phone for the duration of the reset process.
That said, doing a reset will render apps on the SD card unusable, so they may as well get erased.