How do you foster a culture where both product people and engineers feel a sense of accountability to delivering features, without focusing on the negative if we do not deliver?

It is important to focus on outcomes, not only shipping features. We can deliver features and not deliver outcomes, so in terms of KPIs, I tend to focus on ARR, or beta sign-ups, or customer usage of what we ship. Getting teams to rally around major milestones (like a conference where we launch a new product) works well and gets teams excited to deliver to a date. Creating visable, shared dashboards where we monitor and celebrate these outcomes (KPIs) helps drive positive behaviors. When we miss, we must do a retrospective to understand what happened and how we grow from it. This should be no different then any other team at the company.