It looks like the Safety Net is just a conservative asset allocation of stocks and bonds. It looks like it’s set at 30% stocks, 70% bonds.
Is it SAFE? Well, you’re holding investments that can change in value. Both stocks and bonds can certainly go down in value. But holding cash also isn’t perfectly “safe” as it it always losing value to inflation. So you have to decide which risk is better for your situation.
I have no problem with Betterment’s theory on this medium term asset allocation. It sounds like something I would do myself. (Note for very short terms, like money you know you’re gonna spend within a year, I’d keep it all in cash).