Great video. Tesla cars would just be so much better with these, it's kind of infuriating.
(Also the rear cross alert pings from my old (MT Sedan) Mazda 3 Astina '14 is something I miss greatly.)
Having said that, the wipers in my LR3'22 work pretty well and I am not really giving it that much thought.
I keep the area clean and wipe the blades down regularly. I do like how easy it is to engage the wiper service mode to be able to lift the blades; no manual required.
There was a time when perhaps I didn't keep the area that clean and found I couldn't really use cruise control on a highway drive without the wipers going nuts in the dry, fortunately it's been a while since I've experienced that - upgraded behaviour, perhaps.
It would seem like you could sense rain hitting the roof through its acoustic signature using a contact piezo mic. That'd probably have a much larger effective sensing area than the IR sensor.
You don't need a large sensor area or complex audio analysis. A spot with a few IR diodes on the windshield is going to get wet if it's raining hard enough to need wipers.