Can USB charging (via mains adapter or from PC/Mac) of Android tablets be the main, primary method of charging? Or are there design issues?
The USB spec specifies that the power supplied over USB shall be ~5V DC, which is fine.

A single USB device (e.g. your tablet) may draw up to 500 mA of power in USB 2 and 900 mA of power in USB 3.  This is the issue - it's workable but not ideal.

Using a USB cable as a power connector between a tablet and the AC adapter that came with the tablet is a different story - they can do anything they want - but charging from a USB port is governed by the above limitations.