I suspect that there was no possibility of doing enough research to be able to predict what the market was going to say. I imagine at least two of the competing requirements for their slides would have been extreme durability for .40 and 9mm (that's a classic request item from a lot of people) and also slide serrations on the front and rear without too much weight. Of course, weight and durability and "40/9 convertibility" when it comes to modularity are all competing requirements.
And then there is the design itself. I know that plenty of people had to have had a chance to provide feedback on the slide serrations during testing, so I don't think that was likely an issue. However, there's the marketing aspect that comes into play as well. While there are a few people who really will go through each platform on its merits and decide what works for them, the majority of people will have trouble telling one gun apart from another without some visual cues. I suspect that Beretta wanted to make sure that the visual distinctiveness of the gun was retained while also providing slide serrations along the length of the gun.
I think this is a case where there is no way they could have got it right. If they went with more traditional serrations and a slide design, then people would have complained that it wasn't unique enough and didn't bring anything to the table, and they wouldn't have been able to make it distinctive for advertising. If they went with the distinctive design, then they were going to turn people off for being too unique and different compared to what they are used to. I don't think there was a way they could have avoided negative press regarding the serrations.