Roc uses perceus, which is a reference counting model that allows for opportunistic in place mutation if it is safe to do so. HVM is more like a fundamentally new evaluation model that is parallel by default. They are both very exciting, but HVM is much more radical and probably needs at least a few more years until it starts to be seriously practical.
Does it have any support for laziness? E.g. could one define the list of all fibonacci numbers similarly to Haskell's