I'm not sure it if is the best way to recover phone but it works for me.
I had a problem with "stuck on Nokia logo" after roll-back from Android P preview to Android 8.1, and probably I found a solution.
To reboot the phone from Nokia logo You need to press and hold volume up + power buttons.
When display goes black:
a) press and holds volume up + volume down + power (phone can reboot few times without recovery mode but should finally go there).
b) press and hold volume down + power buttons (boot to the download mode) and then point a)
*I'm not sure, probably point a) should work without point b) but I fought with it around 1h to enter the recovery menu and tried multiple combinations.
In recovery menu try to wipe data, after reload wait few minutes (as while the first bootup).
If it still doesn't work, enter to the recovery menu and flash Your device again (with Oreo image). And then wipe data and reboot.
Can you tell me how to flash an Oreo with Fastboot?
I have extracted a nb0 and now have a folder with 70 files.
*Apply update from ADB
Now send the package you want to apply to the device with 'adb sideload filename'
Finding update package
Opening update package
Verifying update package
E: Footer is wrong
Update package verification took 0.0s (result 1)
E: Signature verification failed
E: error: 21
Flashing from recovery mode don't work!
I'm on Nokia logo, can't turn off the phone and can't get to the black screen. How to fix it?