Yes. Right now there are significant functional differences. In the future, maybe. Human Interface capabilities are always going to be the primary limiting factor, and as of now there are plenty of people who still much prefer actual tactile responses to computing to verify interaction. Physical buttons very clearly respond to inputs - except of course when they don't for [insert IT guy sigh here].

Plus, I can tell you for sure that Linux ARM distros are functional, yes, but don't compare to what an optimized PC distro or a dedicated mobile OS on ARM is like. My RaspberryPi with its fully optimized OS and ARM processor can't handle the same thing my phone can do in the background while I browse several tabs on the internet. Is it the RAM? I dunno, my laptop had the same RAM as the Pi until a couple months ago and still could multitask better. Play a game and watch a video and have multiple tabs open for the game. The Pi struggles on certain websites and that's all I ask of it.

Just because it exists doesn't mean it is ready for primetime.