"We genuinely thought we had done a good job," he told the Renault Sport F1 website. "It was the case in certain areas, but we were caught out in three principal ways."
Taffin listed them as a significant reliability problem with the pistons, a malfunctioning part inside the power unit electronics and taking last-moment decisions on development.
Renault's under-powered and unreliable engine had been eclipsed in 2014, the first year of the V6 turbo hybrid units that replaced the old V8s, by dominant Mercedes who won both titles.
"We wanted to make up the gap so badly that we pulled the rug out from under own feet," he said.
"But this particular cocktail of problems did allow us to learn a lot and make sure that the same issues will not reoccur in the future."
Renault provided engines to Red Bull and Toro Rosso this year and will again supply two teams in 2016, with Lotus switching from Mercedes while Red Bull's unit will carry Tag Heuer branding.
Ilmor, the performance engineering company with decades of experience in Formula One and IndyCar, has been drafted in to assist.
"They were able to bring different concepts to the table, such as a different combustion concept, and now we will continue with them into 2016," said Taffin.
"We will continue to develop a lot in-house but with partners such as Ilmor we have a greater bandwidth and a richer experience and expertise.
"We did not see the full extent of the collaboration in 2015 for various reasons, but now they are under our control and we have a clearer direction we should see this more in 2016 and beyond."
Taffin said the 2016 unit to be used in next year's season-opener in Melbourne in March would be running on the test dyno this month and the specification for the first race had been fixed and seemed reliable.
"We are in much better shape now than we were at this point last year," he said.
(Reporting by Alan Baldwin, editing by Clare Fallon)
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