Perfect combination for me is:
Topbox hard (rear) - keeps stuff locked and 100% dry. Good for bulky but light things.
Soft side panniers (rear) - good for stuff that doesn't have to be locked while you take a leak. Moderately heavy/bulky stuff.
Soft tank top box - good for heavy stuff, doesn't affect balance. Can easily be dismounted and carried along at every stop, so OK for wallet, phone, passport etc, but you need to remember to carry it.
Last spring I travelled the North of Italy and at most stops I carried topbox and helmet with me when I had to leave bike.
Soft luggage is cheaper and harder to destroy. It can get torn, but you can saw/tape it. Hard boxes are almost useless if racks or boxes themselves get damaged.