Some apps won't work from the SD card, but the person who made the app didn't know to check whether it would, so it's not prevented from being moved.
Not all parts of an app can be moved.
And each part that's moved leaves a link in internal storage, pointing to where it is on the external card, so you may not save as much space as you thought you would.
Also, backing up a card, formatting it and restoring it will work fine for files - but the apps that were moved to it are no longer installed - because they're not restored to where the links say they are. (Backing up and restoring copies files, it doesn't make a clone of the card.)
Best bet - use a phone with enough internal storage for your apps and leave the external card for what it was designed to do - be a repository for data.