Also, ads can have negative impacts from the thing that's being advertised if it ends up being a scam, dangerous/fake/misrepresented product or malware.
1) The long-term validity and usefulness of the data they are gathering. A user may be relatively net-savvy regarding the use of private browsing, they may rescind their consent on this or that advertising network to track them, their visiting son or daughter might give their device a "privacy clean-up", or they may be ironically so digitally illiterate that when their phone dies or they forget their passwords they just create a new Google account.
2) The legal risks involved in gathering, and particularly exploiting, non-GDPR (or other local law) compliant data.
With non-consensual cryptocurrency mining, both points become irrelevant. Once it is mined, cryptocurrency is a pure commodity, regardless of legality and the consent or future opinions of the person whose resources were employed to produce it.
I know Coinhive originally started like that, for this Polish imageboard where you explicitly had to allow your CPU to mine in order to get premium account time, as an alternative way to just paying for it outright.
Firstly, whatever resources are consumed by ads, resource consumption is not intrinsic to their nature. In other words, the effectiveness of an ad does not depend on how many resources it uses on its target's device.
Furthermore, the resource consumption by ads can be argued to be a real creation of value, in the sense that the advertiser could not make money from running ads on their own devices without any involvement of their targets. Neither is the target being deprived of any profit that they themselves might have wanted. Would the target wish to run their own advertising aimed at themselves on their own browser to promote their own product and/or track their own browsing habits? Of course not.
Compare that to covert cryptomining. The cryptominer could achieve the same result by running the code on their own devices, and paying for the resources consumed. On the other hand, the target is being unknowingly deprived of the profit being generated by the resources that they themselves are paying for but chose not to employ in cryptomining. Would the target like to use X% of their electricity bill, sacrifice Y% of their device's resources, and Z% of its durability lifespan to obtain a certain amount of a cryptocurrency? Maybe.