Flixbus claim you will have free WiFi on all of buses. As I mentioned, in my experience it was OK, I could even work on my laptop, but there are complaints about it in some other flixbus reviews. I guess it depends on the bus.
Think of it this way: the WiFi is minimal. I mean, it is normal that WiFi can fail. If the whole bus wants to watch a movie at the same time, then I'm sure it is not going to work.
Be sure to download all the movies/ podcasts/ apps in your hostel so you don't have to rely just on the bus WiFi.