As for why this is such a big issue, Leung told me that "every new plug, connector and cable now has to be certified to be 3A compatible." That's fine, and when everything plugged is USB-C but when you mix up cables with a C connector on one end, and older A or B connector on the other, adapters and legacy cables should use lower 1.5A or 2.4A charging. Not all of them do, which can cause problems since the charger on that end might not be rated for 3A charging.
For an example of how companies can fail to meet the standard, check out this review of a TechMatte USB-C to MicroUSB adapter:
Specifically, these adapters do not charge the Chromebook Pixel 2015 because the adapters leave the C-C lines floating, where the specification requires a Rp pullup to Vbus to identify the cable as a legacy adapter or cable.
Please see the document named "USB Type-C Specification Release 1.1.pdf"
section 188.8.131.52.4 for a description of why the Rp pullup is necessary.
Please also see Section 4.11 and the following note :
1. For Rp when implemented in the USB Type-C plug on a USB Type-C to USB 3.1 Standard-A Cable
Assembly, a USB Type-C to USB 2.0 Standard-A Cable Assembly, a USB Type-C to USB 2.0 Micro-B
Receptacle Adapter Assembly or a USB Type-C captive cable connected to a USB host, a value of 56 k'
± 5% shall be used, in order to provide tolerance to IR drop on V BUS and GND in the cable assembly.
In other words, since you are creating a USB Type-C plug to a USB 2.0 Micro-B receptacle assembly, you must use a resistor of value 56k' as a pullup to Vbus. This cable does not do this.
Please let me know if there is any more information I can provide about why these adapters are problematic.
If you are a consumer looking for a cable that is compatible with Pixel, do not use this one.
So what can you do? (Other than keep an eye out for reviews by "LaughingMan" on Amazon.) Leung has posted instructions that can help Pixel owners test the specifications of USB Type-C cables they've connected to the laptop, but that only helps after a purchase. Companies like FREiEQ, Belkin and iOrange have all produced cables that Leung found passed his tests. Beyond that, he advises that your best bet for now is to buy first party cables from the likes of Google and Apple.