I suspect that a lot of these chargers are geared towards the loaded touring market, where people are away from wall sockets for a long time, but never really go all that fast. For shorter rides (1-2 days) a USB battery will be no heavier or bulkier than a USB adapter for the hub dynamo, but when you're out there for 5 days and more a charger starts to look appealing.
I don't really do what I would consider high speed descents. On the Bike Friday I would almost never exceed 40-45 km/h on descents. With the smaller 451 wheels this works out about the same in terms of dynamo RPM as a 700C wheel at 53-60 km/h. While I'm sure roadies exceed such speeds, maybe on loaded touring bikes people would keep speeds down a notch or two.
Anyway, those were the speeds that did it for the three models by two manufacturers I tested. The second used a more sophisticated design that doesn't simply burn off excess voltage via a zener diode (which can overheat the guts of the charger) but used a protection circuit that disconnects input power into the step-down circuitry, similar to what's claimed for the SineWave adapter. My third unit was a beefed up version of the second unit, but it still died, so I gave up my quest at that point.
As with so many other products, the proof of the pudding is in the eating. People use them differently, riding on different bikes across different terrains in different weather conditions. What works for some may not necessarily work for others.
My most recent battery purchase was this item: JETech モバイル充電器 10000mAh 2USBポート ブラック B0760 on Amazon Japan for 1599 yen so my kids will no longer steal my Randonneuring batteries (I have three) for use with their smartphones
The JETech battery works pretty well and should last for days at a fraction of the price of a USB adapter that still needs a buffer battery to work well in some road conditions.