HyperNext doesn't appear to be actively developed; the most recent updates I see are from last year, and it can't be used on modern computers. Decker's most recent release was yesterday morning.
There's a fair amount of usage of it on Itch.io, if you are into that indie crowd. I was skeptical of it at first -- the whole 1-bit dithering aesthetic seems a bit too retro-twee, but I find it it is the best Hypercard-alike in terms of functionality -- it "just works" as compared to most Hyperclones that seem more like a proof of concept than a functional program.
Why use it rather than Livecode (aside from the licensing of the latter) or Hypernext Studio?