Are USB hubs that backfeed power upstream bad?
That is super terrible design... but will it break anything? Good question..

If you try to backfeed a voltage into a supplied voltage, the difference in the two will determine the amount of current that flows. So if the supplied current is 5.1V and the applied voltage is 5.2V, 0.1V will cause current to flow into the 5.1V supply, unless it's specifically designed to prevent current from flowing backward. (this would depend on how the switching converter is designed)

So say you have a long cable, and 0.1V of mismatch... that might be around 5 ohms... That is 0.020A, or 20mA. That is enough current to run an LED, but usually not enough to do damage to a power circuit. A much worse condition (as Russell points out) is when the computer is OFF and there is power being backfed.

Most USB power systems are going to have a resettable fuse somewhere around 500mA to 1.5A, so that would protect things in an extreme case, but the real answer is that it depends on the computer, and it depends on the design.

My best advice is to stay away from USB hubs that are of mindblowingly poor design.