Well it should depend on the market, but overall, I think $50 with 15 post requirement is very worth it, although the campaign already ended. The requirement isn't hard and get paid good enough, if there are two campaigns pays $100 with 30 post requirement and $50 with 15 post requirement, I will go for the second one.
There's also Roobet, which requires 15 weekly posts and a fairly high payout of $70 there are no mandatory posts on gambling, so it's worth it.
Honestly, I won't be a hypocrite; I'd choose $100/week with 30 posts because the payout is quite decent.

But if it were $50/week with 15 posts long-term, I'd choose this.

And who knows if there will be next announcements of additional members. Because they seem to be in a momentum and the brand does likes how the marketing goes with their campaign.
There's a possibility of further expansion, but I don't know when.
I'm not sure if it's the most massive campaign ever, but there were also some massive campaigns in the past. As I remember, for example, Yobit, but there was no public spreadsheet, because the counting was automated. I think there was a thread with cheaters and spammers from that campaign, and there were more than 100 of them alone.
Therefore, Rainbet remains the most massive, as there's published spreadsheet data.