They work particularly well after speaking to lots of users (or reading their interactions on forums), to see the language they use when discussing the problem your solution fixes (and using that language in your copywriting).
For most products, Product Hunt is a waste of time. On the other hand, if you're a top 3 product that day, you'll get a steady stream of signups for months, if not years.
I've found it's harder to read signals from ads for early stage releases. You need something to compare to... Then again, maybe a super narrow target with a very clear conversion might yield some useful engagement.
GL!
What channels do they use?
Do you want more users like them?
Can you gain users one by one using manual methods?
How much runway do you have?
Is there something you could do right now that would gain users?
Are you not doing it because it is harder than paid ads and/or Product Hunt?
One advantage of paid ads and/or Product Hunt is they allow the creation of after-the-fact narratives for unsuccessful products. Being very easy is another. Throwing things over the wall is always easier than sales because sales is hard work.
Good luck.
I would only use Product Hunt if you're targeting Silicon Valley audiences. In general even with physical ads consider the network/reach/visibility and who the audience is.