Currently testing Yosemite on a clone of my startup drive.
Epson’s recommended driver for Yosemite is version 9.17 and was released in July (so maybe during Yosemite public beta?) But I don’t think i’m running that version but the previous version which is working fine - I think it’s version 8.64.
LR, PS CC 2014, and printing to 9900 all working without issues. Also running yosemite on my laptop and no issues with CaptureOne 8 on that machine or any other software that I’ve seen.
I don’t think there was any real changes to the printer pipeline, from Mavericks. Not sure why you are getting the annoying message, but surely there’s a fix for that somewhere?
Hopefully someone actually running an HP on Yosemite will confirm compatibility for you. Or try what I (and others) do, use carbon copy cloner to copy your startup drive, then boot from the clone, and install yosemite on it. Run from the clone until you’re comfortable, then just clone the yosemite drive back over onto your original startup drive. Or if you run into problems, just restart using your original startup drive and mavericks.