The Pinion gearbox requires two cables because you are always pulling the gear into place. Essentially, when you pull the cable it shifts a cam shaft in the gearbox to engage different gears. A spring doesn’t work because it isn’t strong enough to pull the gear into place, and would have to be pretty long on the handlebars or transmission to accommodate all of the gears. Derailleurs have it pretty good because they can use gravity and a spring to shift the gear down the cassette block, but even that isn’t strong enough to pull it back up the block. If you want to get into the detail, Pinion has a really cool site that explodes the gearbox into all of its parts and how they work. https://pinion.eu/en/p-line/technology/
On the original question, I have briefly ridden a Zerode Taniwha and it was amazing. The rear wheel tracks amazingly well and the suspension comes alive. The shifting is a bit weird in the beginning, mostly because it isn’t what you’ve trained your body to do. If you think back to the first time you got a bike that had more than one gear and it’s a similar thing. It feels awkward at first and then becomes second nature pretty quickly. There is a bit more drag in the system than what you’d be used to. Based on what I have read, the gear box beds in after a few hundred miles and it feels much better.