Yup - my wife is having the exact same problem with her iPhone 6 for well over a month.
She has an iPhone 5 (that the 6 replaced) and an iPad retina - with iCloud Photo Library Beta enabled on all devices.
The iPad and iPhone 5 have both happily uploaded their pictures and they can be viewed via iCloud on a web browser - so I know the images have been uploaded.
The iPhone 6 though, still thinks it uploading 15,000+ images - and has been like this since switching the feature on (It was 14,000+ but she's taken more pictures since). I originally though that it might be because the iPhone 6 was basically set up as the old iPhone 5 was - via an iCloud backup restore that would have contained all the pictures from the original phone… meaning there would be around 13,000 'duplicates' trying to be uploaded! …but since so many other people are having an issue, it probably isn't that. Besides, I'm sure Apple would have made provisions for the situation where identical images are uploaded from different devices…???
Annoyingly, images from the iPhone 6 are available on iCloud Photo Library - up until the end of November (The same time when the whole web-app wasn't available) - but none have uploaded since.
Finally, if I try and switch iCloud Photo Library Beta off on the iPhone 6, I get a warning that around 1,300 low-res images will be deleted from the phone as it hasn't downloaded the high-res versions. Which is odd, as I instantly switched on 'download all high-resolution images' instead of 'optimize iPhone storage' when enabling. So I'm not sure I want to do this as I have no idea what images it would delete!! 😟