So last week I went from an iPhone 7 Plus to an iPhone X. In addition, I have an Apple Watch Series 2 38mm Space Gray. Both the iPhone and Watch are updated to their latest OS releases.
The Watch worked perfectly with my iPhone 7 Plus, maintaining a consistent connection. But since getting X, the watch does not maintain a consistent connection and is interrupted periodically and immediately reconnected, resulting in shorter battery life. With 7 Plus I had to recharge every other day; Now with X, I have to charge every night.
I've started both iPhone and Watch. I have unpaired my wat and pair it back to X, restored from a backup. Still have the same problems, but I don't notice them half the time. When my clock disconnects and I receive a notification, my phone vibrates and / or gives a sound, so a second later the clock gets me alerted.