I have never had an issue taking Uber in HK, but sometimes it's faster just to grab a taxi (in which case can confirm you generally need cash).

Contactless cards are widely accepted as noted above but many places (mostly local stores but occasionally even like a regional chain in a mall) are "cash or Octopus." To add to the above, you can refill your Octopus via the app I believe if you happen to have an HK bank account linked to it.

I have encountered some cash only situations in stores/restaurants but in that case we're usually talking about remote island villages and such.