It probably depends on how personalized the ads are. It should come as no surprise that targeted ads have been a thing as long as ads have existed. Coke ads during family shows, alcohol ads (you'll be cool if you use our product) on MTV, etc.

There are already a number of algorithms to deliver one ad for a product over another, based on likely demographics. Streaming has greatly increased this, since specific demographic details are immediately and directly available when the ad is shown. I won't get the same as on Hulu that you get.

As for generating ads on-the-fly, that seems unlikely. Few ads are fully generated at all, let alone in response to demographics or the specific viewer(s).

There are also risks when deploying ads without vetting. Some of these viewers will be dressed as Nazis, and will happily share the video to everyone.