Uber didn't honor the upfront pricing! What is the point?! Is it not guaranteed?

Upfront pricing is not guaranteed. Welcome to reality!

First of all, if you are not familiar with upfront pricing, please check this article here: What is upfront pricing?

You are correct.  Uber does not have to honor the upfront pricing under some circumstances.  They have rules in place, but they are usually to do with the driver changing destinations or the route that drastically changed the distance or duration of the trip.

Quote from Uber:

When you request a ride, you agree to be charged the upfront fare when the trip ends. Your fare may increase if you travel to a different destination or make extra stops along the route, or the trip takes much longer than expected. 

If an upfront fare is not honored, you will either be charged the minimum fare or a fare based on the measured time and distance for your trip, including any base fare, booking fee, surcharges, tolls, and other relevant factors such as a dynamic pricing charge. 

You'll always get a receipt for any trip fare. If your fare is different than the upfront fare you agreed to, your receipt will explain why. 

As a result, some controversies arise:

• The Uber price didn't change, even after instructing the driver to take a different route. 
• What happens when the Uber driver goes on a longer or bad route?

Source: https://help.uber.com/h/d2d43bbc-f4bb-4882-b8bb-4bd8acf03a9d

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Uber Driver, CEO of TaxiFareFinder and Unleashed, LLC