Historically, running Ruby on Joyent was an exercise in frustration. Heroku isn't only Ruby these days, but operating as a stable ruby platform is still a requirement I think.
Among other things: Solaris threading libs with crashy bugs when you pop a new thread open. Took quite a while to figure that one out.
It's annoying that the two people that try to tell me that "Rails doesn't scale" are also Joyent customers. I tell them, "no, you can't scale Rails".
It is perhaps a difference in thread creation semantics, or something like that. But, Solaris threading has been rock solid for many, many years.
Not in my direct confirmed experience. pthread lib on Joyent's particularly weird version of solaris was f'd hard.