As Amazon becomes the latest platform to push an ad-supported tier, TV writers greet this retro model with frustration and, in some cases, disdain: “I thought 'Nine Perfect Strangers' with commercials was horrible,” says David E. Kelley of his Hulu show with breaks.
The thing I always noticed when a service places their own ads, is even when there are “ad breaks” on the timeline, the ads don’t always show up there. Or the screen blacks out for a few seconds, then the show plays, THEN the ad would play.
Granted, this was a while ago when I actually put up with that bullshit, so maybe it’s changed by now. But it was done very, very sloppily and is almost certainly a creator’s worst nightmare for the story.
Content providers can probably include chapter markers in their content. I also suspect it's not hard to detect a scene transition. Failing these, randomly placed.
This was my problem with hulu back in the day. Short episodes like Futurama would have a commercial shoved in at like 3 minutes and then again at 10 or whatever, it was obnoxious and shittily implemented.