It really depends on what type of business it is. For instance, a product feature or product should be so frictionless that it allows users to onboard (adopt) easily once they are in the experience. Product marketing is responsible for bringing users to that experience. So it is almost eyeballs vs. activation. In reality these metrics should be shared, and over time broken up where the tech is meant to solve for the customer's need.
Such a great question! When you first set a KPI especially if you are in a new market and/or in a new product/customer space, it can feel uneasy. The best way I have learned is by setting something and tracking it over time, seeing if there is any measurable change. If not start by marking out the customer's journey (no matter who they are) and see if you can collect data on their interactions along the way. This may reveal some hidden trends you weren't yet measuring.
KPIs around delight unless this is your key product differentiator (which is proven to be compelling to customers). Focus on building an intuitive and effective product experience that users would want to recommend to their friends/colleagues. Focusing on the final pieces of polish such as interactions, delight, animations, etc are fluff until you're really providing value to your customers. This is why keeping your KPI or success metrics concise and essential will allow you to provide the most impact to customers.