Majority of the CNS disease are same - difficult to treat - so no one can be so confident to say that it will be 100% success.
PDD is 1st trial so higher chance of failure. Look at ZYNE - great P2 results - was considered slam dunk for P3 and they were talking about registration - but it failed.
So I am optimistic as I do not want to lose money but expect 50%+ chance of failure