do safety charms work inside boss areas like Zakum or horntail? Are you revived on the same spot/map?
Safety charms do not revive you. They just keep you from losing exp when you die. Otherwise it is exactly the same as when you die normally. But yes, they work anywhere. Should you die in a boss, you will not lose exp if you have a safety charm. If you want to stay in the boss, I hope your party has a Bishop with res.
There is an item called Wheel of Fortune that was introduced later in GMS, which revived you in the map you died in (such as zakum's altar). This will never be implemented into Royals however.