The usability or lack thereof of a car's infotainment software is largely unrelated to the kernel underneath. Any modern kernel running on a decent SoC should be capable of delivering an excellent experience. It's a matter of supreme unimportance if there is a Linux, QNX, BSD or NT kernel under it all.
Someone over at ARSTechnica did a review of ICE systems, and if my memory is correct the QNX ones generally had smoother animations, smoother transitions. Having a hard RTOS certainly helps prevent any jittery movements.
I think one if the big reasons why QNX is popular in that space is that there's good support teams that can help get BSPs right, and it's what a lot of other automotive places use too. It means less time setting up BSPs and hardware, more time writing ICE software, and a commonality across the industry which helps with staffing.
There's companies do that (BSPs, support) with Linux too, but they don't themselves own the OS and so are not in control of its technical development. Linus wakes up one day with a great idea, could be a big fork.