All investing services, automated or not, can show their historical performance. They can slice it how they like to tell the story they want. This is not a surprise I hope. :).
As I say, "past results are no guarantee of anything".
You can't really measure platforms because you would have to account for the choices all the members make.
This is pretty unlikely, but let's just say all of Betterment's customers are hyper-conservative bond investors. Their performance is going to be terrible compared to some other place allowing high-risk ETFs in a year when that happens to be both popular and a good idea. It's not because the investments were bad.
I don't know that there is any truly objective measurement like you're asking for.
What really matters is to be comfortable with the firm, how they do business, how their statements look, their attitude toward you and the forthrightness of what they say and mean.
If you're interested, we should talk a bit (for free). I specialize in education and collaboration that includes the professional financial stuff done without the high cost. And I give customers the same professional tools that are usually limited to the yacht-owner crowd. Let me know!