That might work for a normal commercial product - in such case you should generally only announce any dates only when you are really sure you can meet them. Or post factum: "Hey, we released X!"

On the other hand - we are talking crowdfunding. For crowdfunding such behavior wont do. Anything other than frequent updates and maximal openness won't do - especially when you hit any unexpected delays. Backers will generally be quite okay even with severe delays as long as you regularly keep them up to date. And you risk making them very angry if you keep them in the dark even for small delays.

As the Jolla Tablet crowdfunding campaign was half crowdfunding and half Sailfish OS/Jolla PR then I think they really really can't afford making their backers angry - so maximal openness and frequent updates are IMHO the only way to go now for the Jolla Tablet.