Yeah Siri can be pretty useless. I mean, when it does work it's great, but sadly most of time it's like arguing with a half-deaf retard, so it can be actually more distracting than just using touch.
Now touch interface in general has many problems for car use:
- you can't feel the controls with your finger without looking
- no haptic feedback when you hit the control
- you have to be careful not to touch the screen at the wrong place, because you will trigger something unwanted - that's not easy to do while driving, more so if the road is not perfectly smooth
- it's not predictable enough - controls are changing position on screen, so you can't use your muscle memory to learn the place where to hit the right button without looking
- it's asking too many questions
- too many menus/clicks before you get to the desired function
- lacking gestures, which you could use without having to hit the exact on-screen control, which is hard to do even if they're large. For example something like slide left/right for prev/next track, double touch on the left/right part of the screen to forward/rewind 30 seconds, slide with two fingers left/right for prev/next album/playlist etc. (forgive me if Caplay does have any of this, it's been a while since I tried it)
- it needs better system for quickly locating items in the lists without having to scroll through them, since scrolling through big lists is a very bad idea while driving
Basically the interface needs to be as intuitive to use without looking at it as possible. It doesn't even have to be large, just intuitive. Sadly Carplay, at least those few times I tried it felt like it was designed by people without driving licence...