I wouldn't be 100% sure that the Note8 will have a FPR underneath the screen unless they were absolutely sure there were no issues. I read recently that Apple has decided or is leaning on scrapping the idea of the FPR under the screen for their next phone. It sounds like a lot of work still needs to be done to perfect this.
That's reasonable what you say and I agree. What is not reasonable is attempting to rush this new feature onto a next model phone then having to cancel and compromise late in development the placement of the fingerprint scanner.
Samsung should make a decision NOW regarding the fingerprint reader. If it's nearly developed and all the bugs ironed out, then the Note 8 would be a great phone to feature it on. If it's still being worked out, then redesign the rear housing and put a conventional scanner in the middle, lower down like all other OEM's.
Samsung should not repeat this late compromise on such a critical phone as the Note 8, considering this is the successor to the failed Note 7. Every part of the Note 8 will be heavily critiqued because of the Note 7. Samsung cannot afford to have a badly placed fingerprint scanner.