I tried out mining and found it very disapointing.
With companions/Skills/Buffs and Hirelings i got 15 Prospecting and 10 engineering.

Mining/Harvesting Salt at a location with good yield expectation will give me 5-8 Salt, while costing 4 Token, at least a complete day(Supplies+Upkeep), a modarate chance for accidents with costly preparations(masks, Potion of Quelching) or a high chance without, it will reduce Vigor/Moral and more chance for accidents during camp.
At Augur i can by 2-3 Stack of Salt for 5 Bross, then it raises slowly and i have several faction offers for 5,5 Bross/Stack

Mining is a bit better, mining at a location with good yield expectation give me a mixture of metals beetween 5-15 Bross but on average it's just enough to give me my money back if i had no real accidents.

You need some money/equipment/skills and connection to start mining at all, the areas are not completly save so you need some defenders too...

Do i miss something ore is mining/harvesting mostly useless?

The main reason to do either salt or metal mining appears to be the quests related to them that you can get/complete. Which are some of the more interesting ones in the game.

Profit-wise, it's pretty bad.

If salt-mining, just go with an all-slave crew, don't bother about protection and work them to death. That seems to get the best results, as you don't seem to actually lose that many slaves. It also seems like getting the basic protection (masks) is good enough, I haven't really noticed any difference between using masks and using both masks and the potion (it's possible the potion reduces dehydration? But that status effect seems to disappear almost as soon as it arrives anyway, it's the overall energy and morale that suffers - not sure if it reduces those losses?).

The one plus is that I believe the whole token system is broken at the moment - it seems like you're only deducted the tokens for the first day you arrive at a site. If you keep mining the same place for a few days, you don't appear to be charged again. So that's at least one possible saving (albeit a minor one).

For metal mining, working them to death doesn't seem to work since that usually results in a collapse which halts all mining for the day. For that I think it's best to go for a more steady approach.