You'll have hard data on your own products, but you can still only make guesses about your competitors' products, your complements, and other variables in the marketplace. Joining a company doesn't eliminate uncertainty, it just shifts the uncertainty from that company's numbers to everyone else's numbers.
A lot of data is kept strictly confidential; unless there's a mole somewhere, a market research firm isn't going to have it. Many of these estimates are made by applying data that you know about your own company's operations to public information available about your competitors.
