SEO by itself carries a lot of risk, a lack of easily monitored metrics and a general lack of certainty. SEO success is at the whim of Google. Whether it works or not will vary massively based on link building strategy, link quality, link velocity, on-page factors, your niche, your selected keywords and your ability to keep up and adapt to the monthly updates that Google tends to make.
The situations that you want to avoid include using the wrong strategy and be penalized for that or using a sound strategy but picking the wrong (non-profitable) keywords.
On the other hand, AdWords, whilst generally "more expensive" is inherently less risky. You can track it daily, pull out reports daily, rapidly turn keywords on/off etc. In other words, AdWords is a great way to "test" and "prove" that certain keywords have potential for profitability before going down the uncertain path of running a successful SEO campaign.
As a general rule, if a keyword in Adwords drives sales and/or leads (conversions) then it is safe to assume that an SEO campaign will help you to "win more" and scale up those results by an order of magnitude.
This approach of using both Adwords and SEO also encourages businesses and agencies to focus not just on vanity rankings but also on conversion rate and getting the fundamentals right.