I've needed to transfer files between an iOS device and an Android device pretty regularly lately and it's simply impossible, I've literally been waiting months to transfer a few files because the person who has the files has an iPhone and has difficulty using any method other than Airdrop (e.g. Dropbox.) Which I view to be an intentional design decision by Apple. WiFi direct would be fine, Airdrop is proprietary and useless here.
Airdrop is something I use 10x a week.