What you don't build matters as much as what you do. The best product work I've done involved saying no to things that sounded right but would have cost more than they added.
Both are necessary. Quant without qual is pattern-matching in the dark. Qual without quant is n=1 with good storytelling. I've found the best product bets sit at the intersection.
Real input, genuine decisions, accountability for both. I've seen "collaborative" used as cover for avoiding hard calls. That's not what this means.
The best engineers and designers I've worked with have caught things I would have shipped. The relationship is the product — if it's adversarial, the output will be too.
For the user, the product should feel simple. For the team, the problem should feel clear. Both require the same thing: doing the hard work of understanding before you start building.