It's hard to take these "YouTube is popular due to monopoly" arguments seriously when the competition can't even get the basics right.
Nebula isn't free. Only invited creators are on there. Seems like they're clearly focusing on a niche.
I also had issues with Nebula at first. It's been steadily getting better. When did you try it?
I think now it probably performs more reliably than Youtube + Firefox on my Android.
I feel the same about a lot of online shopping. In Germany people often moan about Amazon and while it's has it's share of issues, the competition is often so bad. Really slow processes that feel like someone adopted a "submit order via fax" process slightly for the web, horrible web sites, sometimes next-to-non-existent customer service. No wonder the alternatives aren't taking off as they fuck up the basics before we even get to the point of starting to compete.