I’ve had my own startups and worked with tens of others. I don’t know anybody who has had good experience or ROI with paid ads. There is something “emperors new clothes” about the whole industry where we all play the game but nobody admits they just aren’t very good. Yet we all keep paying the Google bills thinking it’s something you have to do.
What is 'not really good'? We had spectacular ROI on adwords ads. BUT, and the article mentions this somewhat, not for tech/freemium saas products. Developers avoid ads like the plague. For consumer stuff that are just immediately paid (there is no free; you pay or you leave the landing page) it works incredibly well in our experience.
When there is a slower sales cycle and a more complex or less transactional purchase then it all becomes much more vague and significantly harder to find ROI in my experience. The ad-click is likely to be one of a hundred factors compared to transactionally selling a widget.