Ads will play on some shows you DVR through YouTube TV according to the latest details about the service. While it was already known that there would be ads for live content, ads for content that ends up being recorded using YouTube TV's Cloud DVR feature was thought to allow the viewer to skip the ads. As it would happen though it seems that DVR'd shows, quite a few of them in fact, will end up including ads as many shows will have on-demand versions of content available soon after they air. This means if you DVR a show and plan to watch it in a day or two, the on-demand versions of that show will already likely be available, and these will contain ads that viewers will have to watch.
According to YouTube TV the deals they have with networks will require them to show the ads and keep viewers from skipping them which means users will not be able to fast forward past any of the commercials. This does only apply to on-demand content though. Not all shows will have on-demand versions of content right away, and viewers will still be able to fast forward past these ads so they won't have to watch them as they'll actually be able to watch the version of the show or shows that they recorded.
This particular arrangement of deals that YouTube TV has with networks like NBCUniversal, Walt Disney Corp., and 21st Century Fox is a new kind of deal that isn't a part of other services, and it's typical for cable network operators to offer the option for subscribers to make a choice in watching the on-demand version of content or to watch the DVR version that they recorded, thus giving them the ability to fast forward past ads if they want to. There haven't been any terms of this part of the deal mentioned nor is there a complete explanation of why YouTube decided to agree to this as part of the deal, but it reportedly has some part to do with cable networks proceeding with caution in striking deals with YouTube. If this will ever change is unknown but it looks like for now the terms will stick, and those who are already subscribing or are thinking about subscribing will have to endure.