Any kids?
Personally found the Untrainable Dragon show meh at best. The acoustics in the theatre aren't the best, which made the singing (which is a big part of the show a little hard to hear/follow). Storyline is pretty basic, acting was pretty cheesy, someone else mentioned predictable storyline and thats a great way to describe it. Seeing hiccup fly over the theatre was pretty neat though.
Flipside was I really enjoyed Cirque Arcanus. It was unique/different for a theme park stage show. Theming of the waiting area was cool, and the pre-show was a great setup for the actual show (minus being packed in that area like sardines).
My kids would like Untrainable Dragon show more, I preferred Cirque Arcanus. Couldn't tell you anything about Fantastic Beasts, and didn't expect to like the show, but I enjoyed it. I watch HTTYD a lot with two boys in that age range, and thought I'd love the show, but it didn't hit for me.