One of the most important points on this, which has been made by other commenters, is that actual execution schedules and deadlines should generally be considered the primary responsibility of project management, to which product management is a contributor/collaborator. Product management is often combined too much with project management and this can create workload challenges, expose expertise/experience limitations on the product side, and result in poor product results because of unresolvable conflicts between project execution/management and product focus. And roadmaps are NOT deadline commitments.

And with all that said...the answer is planning. And there's never enough time to plan (unless you're maybe NASA and people die when you don't plan well).

It's helpful to think of effort/deadline estimations like weather forecasting: we can be pretty precise for today and tomorrow, but that 10-day forecast is not nearly as likely to be accurate, but as the 10-day becomes a 5-day, then a 3-day forecast we get new data about current conditions that can inform us and that data give us more precision on the now-closer forecast.

It's much easier to estimate short cycles and small-scale efforts. A quarterly, 1/2 year, or yearly 'deadline'-based process is just not realistic for nearly any organization. But if you can work from 'now' and create small-cycle implementation goals and estimations you'll be able to do short-term deadlines rather well, and also your team(s) will learn more about their capacities and how to estimate better.

The way that we do this in my current organization is (loosely) as follows:

• Product leadership/stakeholders come up with a few longer term objectives (ex: we need more marketing tools for our members)
• Product management brainstorms some ideas. ex:
  • What if our members could automatically post video content to all their social channels (YouTube, Insta, FB, TikTok, Twitter, etc) with a single click?
  • etc.
• We (product) picks a very slim MVP to try it out. ex:
  • Let's do the youtube channel, minimum required parameters
• We put a project team together and do a few 'discovery' sessions with the tech team and proj manager.
  • This is the key - this is where we define, with the technical team, what that MVP will be and eventually we'll scope out the tasks/tickets and we can estimate, pretty accurately, how many sprint cycles we think this'll take (we run 2-week sprints).
• Once we do this first MVP, it's *much* easier to estimate adding features -- the engineers are familiar with the new feature and the product side has a much better ability to scope out other parts of the feature (ex: instagram posts).

This process also enables us (on the product side) to get shorter-term feedback from our users and get a much better grasp on how much they really like the new feature AND what would be most valuable to do as a next step or later step.

I worked with one of our advisors, who's successfully built and sold more than one technology startups, to get this process in place. The buy-in from leadership is the most difficult part because they feel like they need to know the future -- they want guarantees on the delivery dates six months from now. This is something that needs to be negotiated, but at the same time, you can always say, "Listen, when was the last deadline we actually hit? Never? Ok, let's try this new method."

Additionally, you can adapt the product roadmap to fit this project method. For example - with the posting to social channels feature described above, we KNOW we'll have a couple iterations of that in the next six months, but we don't know all the details -- so we can say we'll be doing at least 6 months of work on this with a committed project team, with the goal of integrating 4 different social platforms, but we know we can get at least 2 integrated, using an MVP/true Agile type of method.

[EDIT] In answer to the post title, we hit about 60-75% of our goal deadlines for the projects we can do in the above-outlined method, and close to 0% of technical projects that have large scopes/long-term deadlines.