I've spent far too much time trying to diagnose these sorts of things. In a nutshell, the problem isn't 120V vs. 240V. It's about all the different modes of USB charging.

USB chargers, by default, put out 1A / 5VDC. There's sort of a standard kludge that jumps this up to 2A / 5VDC that's supported by everybody-but-Apple, who does the kludge differently. There are a small handful of USB charger adapters that are "universal", supplying >1A to both Apple and non-Apple devices, but it's exceptionally difficult when buying a charger to know whether you're getting the right thing.

To make matters more complicated, some Android devices are now rolling out Qualcomm's "quick charging / turbo charging" that can crank the power up even higher when the battery is lower.

The proper future answer is USB 3.1's "type C" connector, which goes all the way up to 100W (gory details). Hopefully, they'll have standardized branding and whatnot so you can know whether any given charger supports whatever power ratings. The rumors say the forthcoming MacBook Air will have it.

I really hope we get this done right. I fear we won't. Already, there's the new Nokia N1 tablet which features a "USB type C reversible connector with USB 2.0" Sigh. I don't know exactly what that it, but it's something of a harbinger of "not quite" compliance with various future devices until things shake out.

(Side note: I wanted a USB 3 hub for my Mac with a zillion ports. After much shopping on Amazon, I ended up buying an Anker device. Lots of ports. Mostly works, but it identifies itself as a USB2 hub, and some devices, like talking to my Android phone, don't work very well where they work perfectly if I plug them directly into the computer. Again with the "not quite" compliance.)