There are many types of hybrids. There're regular Prius type that has been proven very reliable (the most reliable car sold in US are Prius), then there's plug in Prius or Volt that are also very good if done correctly (although the traction battery would wear out like an EV over time, and expensive due to size, like a real EV), and then there's "mild" hybrids that are barely scratching the hybrid itch without doing anything.
The "mild" hybrid I have seen, some are just using a super capacitor (i.e. early Mazda) to store some small amount of charges in between stop and go, and would not be able to hold enough charges between downhill and uphill, it is almost a joke and it resemble more of a stop start system than a "hybrid".
I don't know what Tahoe / Yukon / Escalades have but if they are a mild hybrid it is a waste of money. If it is a real hybrid like the Prius then it should have an electric motor / inverter / battery that can support almost 1/3 to 1/2 of the total acceleration and deceleration need to be useful (for Tahoe that would probably be at least 100hp), and that would reduce the engine size need by the same amount, and the downsizing of engine would be the main reason for the overall mpg increase.