Dan, that's an interesting point. As an agent you're taught not to make promises about market performance for the very reasons you describe. As a financial advisor, you also cannot control the market and are even more highly regulated, so the only way to protect yourself is via comprehensive disclosures and disclaimers (see all the fine print at the bottom of any investment offering for an example).
If you're both the agent and advisor, there is a potential conflict of interest if you get paid a commission for selling them the house. That must be disclosed. And no, you can't control the market, people do lose money on real estate, so that must be disclaimed.
Financial advisors have the added responsibility of suitability, which means only recommending products suitable for the client given their age, experience, financial situation and tolerance for risk. Real estate agents aren't required to know this. They just need to know if you can get the money, not whether or not it makes sense in your greater financial picture. So, yes, combining the two might be valuable, but the trick is to stay compliant with multiple sets of regulations at all times.