Honestly, I was super pleased that Ubisoft had not just a new IP to announce, but a playable demo. I understand why companies announce an IP with only a CGI trailer--or even just a studio name and concept art, as Square Enix did at their presser. Still, that being so common makes me appreciate when a publisher can drop such a bomb at E3. So for this and all the reasons OP stated, kudos, Ubisoft Montreal.
It really depends on the game, games SE publishes/makes are usually very story driven en they know how to make the gameplay work. This game is a new gameplay, new control scheme, they wanted and maybe needed a fresh pair of eyes and hands to get feedback on things they even forgot to fix/improve/add. And it helps with balancing in the future. So for this game, this was the right way to announce it.