The Planck would be a lot more ergonomic if it were split down the middle.
With conventional keyboards, you don't really have to keep your hands perpendicular to the keyboard, but with the Planck you have to, so you can hit all of the keys - otherwise you will get some weird finger movement which will probably hurt a lot in the long term.
I consider only the ErgoDox, Kinesis Advantage, Maltrons and other split designs more ergonomic and more beneficial to ones wrists and hands because the user doesn't have to bend their wrists in an unnatural way to operate the keyboard, like with the Planck.
However, ergonomics is about what feels comfortable and natural, so to some an ErgoDox may hurt them and the Planck may be the way forward for them.