Most of the friction between product and engineering is not a personality problem or a process problem. It is one missing document, which everyone assumes someone else owns.
The document is not a spec. It is a page that says: this is what we are building, this is who uses it on day one, these are the things it will not do, and these are the open questions with names attached. Two hundred words is enough. What matters is that it exists in one place and gets edited rather than re-explained.
Teams that write it argue earlier and less often. Teams that skip it hold the same conversation four times, each time with a slightly different cast, and then discover in QA that two of those conversations reached opposite conclusions.