I just returned mine and went back to my 6S+
Battery: X has a larger battery than a 8+ yet the battery doesn't last as long even though it supposedly has a more efficient screen. Multiple tests done on websites and user submitted battery stat screenshots show this. X= 7-8 hours use, any Plus model is 10-12+ hours.
https://image.prntscr.com/image/Ks1FqvpsRPafKbjyba4ICA.png
Screen: The screen, while looking good at a perfect angle. Changes colors if you're slightly off axis. My old LCD Plus looks better when side by side just sitting on my desk looking at the same content.
Also about the screen, looking at it for 4 days gave me a 4 day headache. My eyes were hurting and I could not focus. Switched to my Plus, problem is gone.
Screen Size: The screen makes you think that it's bigger, but it's just a trick. There is so much wasted space that in some cases the usable screen area is actually smaller than the 4.7" iPhone 8. Coming from a Plus model, it's a huge downgrade in screen real estate and that's a big sacrifice to make for a phone that doesn't offer anything new or better in comparison to a 8 Plus.
Face ID: This is where Apple tried to solve their own problem, rather than thinking about the consumer. They couldn't fit a fingerprint reader anywhere so they added in Face ID. While it did work surprisingly well and I never had a failure. I felt as though I was trying to work for the phone rather than the phone working for me. I found myself adjusting angles, holding it just right while laying in bed, can't use it on your desk without picking it up... It was not a replacement for Touch ID, which is faster and far more convenient.
UI/UX: The software sucks. Plain and simple. Whoever designed this and thought it was acceptable is an idiot who doesn't know anything about UI or UX. I can go into detail if you like, but just know it's a step backwards and more of a process do to simple things like closing an app or shutting the phone down.
I can probably think of a few more but I think those are the main points that effect most people. The rest is just nitpicking because I expect a fluid and sensible 60 FPS experience and Apple hasn't delivered that since iOS 6... and it's not because of the hardware we have.