Wait, the dementors used to have arms that would reach out?
Don't think they made it to public previews in Orlando. They are stationary arms and hands affixed to the rest of their bodies, but the swift movement and swaying fabric arguably gives the illusion of more animation than there is. The Dementors in Hollywood as well as (AFAIK) Osaka and Beijing all have the arms.
As for the scariness question, the bar here is inarguably Revenge of the Mummy. I know it's been twenty years (gulp), but that was aggressively marketed as a "psychological thrill ride" that would challenge many fears, ranging from darkness to death to insects. The ride itself, when you break it down, is also fairly intense in nature. You are sent to your doom on a cursed movie set, where your one ally has been bound before being basically executed by the antagonist in front of you. Imhotep, which it should be emphasized we mostly see only in gooey glory, threatens to kill us if we don't offer our souls. We are attacked by man-eating scarabs (although the evidence of their destruction has been sadly removed post-refurbishment) before Imhotep attempts to make good on his threat. From there, we are attacked by a variety of mummy warriors before another victim - a hapless ride attendant - is murdered gruesomely just out of view.
Yes, we're all used to it now, and a deterioration of show (particularly that first scene) as well as fan embracement of the narrative's sillier qualities (the last scene) have made it seem less frightening than it probably did in 2004. But Mummy, along with arguably the Reign of Kong queue pre-scare actor removal, is the current ceiling for "scariness" in a Universal ride.
I think it is quite possible that Monsters Unchained will play in similar levels of territory or even eclipse the established bar in some areas, but I wouldn't expect much more than that.