Telltale has not failed us yet, but I simply cannot imagine a good Back to the Future game. (this does not mean I'm not going to buy it asap)
For one thing, the entire movie series is really.. nevrous. Even though they literally have all the time in the world, they are always in a big hurry. Adventure games cannot be in a big hurry.
Next, anything short of perfectly realistic models will look really really wrong. Now, I'm not saying Telltale can't do this, but I have never seen a video game model look exactly like, or close enough to the person they're supposed to look like. (If you have an example, please show!)
I have no doubt that Telltale will make me realise how wrong I was when I play the first episode, but right now I just can't see it.