I don’t know about an alternative per se, as youtube’s whole thing is user generated content, which means alternative-wise PeerTube and it’s ilk are more fitting, though far lacking in the depth, breadth, and wealth of content found on the former. Nebula is almost the opposite of that, being a more curated catalogue of content in exchange for being a paid service. Note, I myself don’t have Nebula yet, but I do use something similar (Dropout.tv) Ultimately I think they’re just fundamentally different use cases, and fit more like an alternative to streaming services than youtube including the faults that come with not having a massive multinational company backing them for years, as does PeerTube.
also streaming services tend not to have comments, which can halt discussion, though that can be remedied with forums, like how youtube creators used to post their stuff to subreddits