My only complaint so far, and I don't know if this is an iPhone 8/8+ issue or iOS 11 issue, but my bluetooth won't stay connected to devices such as my car, portable speakers or my headphones. The bluetooth keeps disconnecting and reconnecting every 1-3 minutes over and over and over. That is kind of annoying, especially when I'm trying to take a call in my car and the call keeps transferring from my car speakers and mic back to my handset back and forth endlessly. Other than that "minor" super annoying issue, the device is great and I have no regrets no waiting for the X.
I have one bluetooth issue and it is very minor in the grand scheme of things. I cannot get it to maintain a connection with an app that I use with my radar detector. It sees the connection, says it's connected but doesn't register. I've seen a couple of other instances of bluetooth issues reported, so maybe it's iOS related?
I thought I was going to have another issue with a bluetooth headset I use for business calls, it did just what you described. I wiped out the pairing, re-paired and it's worked ever since. All other bluetooth devices I've got, so far (knock on wood) have been able to keep and maintain a connection.
It's likely safe to assume you've blown away the pairing connections with these devices and attempted to re-pair them?
Hope you get this sorted out sooner rather than later.