For everybody who so far has publicly said "it's unplayable" on their phones, there's been an equal number who have said it's much more usable on their phones.
Most of the people who have said that they hate the new UI have done so without listing any reasons. When pressed, a few have responded along the lines of "I don't like the colors", or "I have to hit different buttons", or something along those lines. In other words, trivial ****.
There are some bugs. It's seems like they are being addressed promptly by WIS. Though I will question the decision to push this out on a Thursday rather than a Monday, under the assumption that they're not going to have full staffing over the weekend to address the additional bugs that are being uncovered. Two "dead days" of support after a massive overhaul of the UI is not going to win points from those who are upset.
Overall, I see this as a foundational change. Some time ago (a year ago, maybe?), they upgraded the hardware that HBD runs on. Cycles run much faster now. Everybody cheered, because it was a noticeable improvement. Then, a couple of months ago, they made another upgrade to the software that the game is coded in, so that they could position themselves to support and make updates to the game with more current software infrastructure. Everybody "ho hum"ed that change because there was nothing "visable" to the user experience. Now, they respond to one of the most prevalent "demands" from their user community, which was to make the game more mobile friendly. I was assume that the old UI could not be adapted to mobile because it was written before the prevalence of iPads and tablets, and when the majority of people had flip phones and not smartphones, and playing a game like HBD on a handheld device was not on anybody's radar. So they had to come up with something that was completely new, but that retained all the old functionality and same overall layout with respect to menus.
My guess is that this is just another step in the evolution of the game. Bringing it forward from 2006 technology to 2015 technology. As I said in the last paragraph, it's foundational in that it's positioning them to evolve the game even further. And I would assume that should they actually do the marketing they say they're going to do to bring in new customers, that a lot of these new customers will be using handheld devices as well as desktops. Seeing this version as their first experience is likely to be a lot more attractive than seeing the old version as their first experience on their smartphones.
Like I said earlier, some people hate change, period. They want things to stay the way they're used to, forever. As I also said earlier, I think in another thread, some people will always find something to complain about. If you stand on a busy street corner handing out $100 bills to people passing by, somebody will complain that the guy in front of them got a crisp new $100 bill while they only got a wrinkled old $100 bill. You can't please everybody at all times, no matter what you do.