Tuna, chazal blame noach for not davening for his generation, hence the mabul is called “mei noach.” If he was an oness, because he lacked zchus avos (which is arguable – he could have davened bzchua adam harishon) then he would not have been criticized.
Interesting to note that the beis yosef says that noach was an “ish tzadim” and a “tamim” “in his generations” – he says that the nain chait of the dor hamabul was znus. We see from yosef hatzadik that one is called a tzadik if he triumphs over the YH for znus – noach was an ish tzadik in that dor. In the dor haflaga, the main chait was in lack of emunah – the opposite is a tamim, like tamim tehiyeh. “His generations” refers to those two doros which he lived in.
The beis yosef uses this to explain why Hashem said that He saw noach as an ish tzadik lifanai – what happen to tamim? According to this, it hadn’t happened yet.