I tend to use youtube-dl to grab what I want while at my laptop and sync to my phone so that I have things to watch while about. Not a perfect solution, but works for me for now.
As I mentioned in the other [1] thread, you can use this android app [2] (which uses youtube-dl) to download directly onto your phone. If you're on iOS, sorry :(
An HNer posted this, his creation (https://videocyborg.com), in the past month. September happens to be a month where I had to endure a slow home connection most of the time so, when in the odd cafe, it was nice to have the service so I could watch videos later (at home).
I use youtube-dl as well, though I don't sync that to my phone. I need to set up a more automatic mechanism to sync files to my phone when my phone and laptop are on the same network or in the same vicinity.
[1] https://news.ycombinator.com/item?id=12595973
[2] https://github.com/zeronickname/VideoDownloader