Here i got exonys version of samsung galaxy s8 64 GB running with android 7.0 stock rom.
I have set my notification settings to sound and vibrate. Many times when a message arrives , SMS or wapp , my phone would just vibrate for a fraction of a second and either won't play notification sound or play it for less than half a second.
What should happen , that happens occasionally , is vibration and notification tone should play for atleast 1 second (it did on my old s3).
Also it gets worse when phone is on sleep mode , you might get barely a millisecond of vibration and no sound.
It's annoying and requires me to manually check the phone multiple times a day to make sure i didn't miss any important messages.