This mornig, European time, I surprised myself hitting like a moron on Safari's reload button, desesperatly trying to display the local Apple Store page to order an iPhone X. When it finally came up at T:08, delivevery date did already slip to 21-23 of November.
So whatever effort you put in keeping your application up-to-date with latest devices, you'r still facing that some customers will have it running on X while you can't validate your work.
There was a similar struggle when iWatch first came out, and then Apple give us a chance to get a device only a bit latter than the launch date. I don't see anything similar this time.
Even if an application sells several hundreds of thousand $ / year on the store, developer still is nobody.