Honestly, if SWI Prolog serves your needs, use it! Scryer Prolog is still very rough on the edges. However, even with that, some things already make Scryer interesting, like string handling, which is more natural and integrates very well with DCGs, and standards compliance. Scryer passes all ISO syntax tests, and also is one of the few systems that implement dif/2, freeze/2, or even length/2 correctly according to the drafts (this was shown on the meetup, SWI for example failed on all those 3 things). Also, clpz is being developed only taking into account SICStus and Scryer, since they implement the same Attr Var interface (SWI has another one).

I don't agree that there's no progress. Other Prolog systems were started in the 20th century and they received funding from universities or they've been commercial. Scryer has neither of those things. For the most part, it was developed in free time. It needs to form its own community of users that will improve the system. That's why these kind of events are so positive in my opinion.