For me it always vibrates really strong everytime it successfully unlocks using touchid on my 8. It’s super annoying I always think I received some notification during the unlock (that happens to me a lot). Unfortunately I think it’s a new feature since when it does not recognise the fingerprint it vibrates twice. And there is no vibration when unlocking with a passcode so I’m thinking they intend it as an indicator of an unlock with touchid.
It happened to me a few times though in the past that I had my phone in a hand and as I was walking I received a notification, ignored it but one of my fingers was on the home button, the phone unlocked and I accidentally deleted some of my apps or texted someone a few weird emojis. So it might be helpful, but they should really tweak the vibration and maybe provide a switch.