While it is true you could try to use each to serve the purpose of the other, it quickly becomes clear that they are addressing different needs.
A Canvas is especially helpful when you are starting a new business, or creating an entirely new product line. In this situation, you have many different risks, including validating your value proposition, figuring out how you intend to make money, how you plan to get this product out to your customers and sell to them, how much it will cost to produce this product, what will you measure to track your progress, and not to mention determining if there is a market there to sustain this business.
For decades people would create business plans to try to highlight these topics and how they intended to tackle them. But many people, including me, have written on the many reasons why those old business plans were often more harmful than helpful.
A Canvas is intended to be a very lightweight tool to call out these risks early and encourage you to tackle them up front and not just think everything is about coding. I much prefer the Canvas to old style business plans, but I have also observed that many startup teams still spend too much time on the Canvas and keep postponing that pesky little problem of coming up with a solution that people actually want to buy.
In contrast, when you have an existing product, we are constantly considering new projects, new designs, and new features, and an Opportunity Assessment is designed as a very lightweight tool for highlighting the risks of the new effort.
In the case of an existing product, the majority of a Canvas is irrelevant. You already have a distribution model. You already have a monetization strategy. There is already a well-defined cost structure. You are simply trying to create more value in your solution.
The key questions in an opportunity assessment are: what problem are you trying to solve; who exactly are you solving this problem for; and how will you measure success? You can see how you might also be able to use this for framing the work of creating an entirely new business, but I would argue that the Canvas does a better job of highlighting the most common risks.
Normally we assess this ongoing product work, and we maintain a prioritized list of assessment which is often called the Opportunity Backlog. During Product Discovery we are seeking good product solutions to these opportunities, and when we succeed, the work is described on the Product Backlog.
So if you’re starting a company or even a new product line, consider a Canvas to highlight the key risks and frame your work. Once you have an existing product that you are investing in, consider using an Opportunity Assessment to highlight the key risks and frame the ongoing product discovery work.