A single word or phrase for something that necessarily causes both harm and benefit?

Solution 1:

From thefreedictionary, it's a...

two-edged sword - something that offers both a good and bad consequence

Also double–edged sword (M-W) - both versions are equally common on both sides of the Atlantic.

See also it cuts both ways (it affects both sides of an issue equally).

Solution 2:

Necessary Evil — Cambridge

noun Something unpleasant that must be accepted in order to achieve a particular result

"I think he regards work as a necessary evil."
"Most Americans accept taxes as a necessary evil."

Solution 3:

That can sometimes be called a mixed blessing:

something that, although generally favorable or advantageous, has one or more unfavorable or disadvantageous features.