Mark Gurman, Bloomberg:

Apple said in September that the iPhone X and iPhone 8 could be charged wirelessly. It recommended charging hubs from Mophie and Belkin, an unusual move for the consumer-hardware specialist. Apple also announced its own AirPower charger, but said it wouldn’t be released until 2018.

And:

Company engineers have been toiling away to address problems. One challenge is making sure the charger doesn’t overheat. Another is the complexity of the circuitry, according to people familiar with the device’s development.

And:

Unlike wireless chargers on the market today, the AirPower is designed to charge three devices simultaneously: an iPhone, Apple Watch, and AirPods with a still-to-be-released wireless charging case.

And, the point I think is the heart of the problem:

Apple also wants users to be able to place any of their devices anywhere on the charging mat to begin a charge. That ambitious goal requires the company to pack the AirPower with multiple charging sensors, a process that has proven difficult, the people said.

If you take apart a Qi wireless charger, you’ll find a coil of fabric-coated wire, the induction coil behind the physics of wireless charging. That coil is always round, and the chargers you buy are typically round as well, keeping the case design at its smallest form factor.

Here’s a video showing a tear-down of a Samsung Qi charger. Jump to about 3:58 in to see the coil.

Apple’s AirPower charger is oblong, not the same shape of the existing, circular Qi chargers. Some physics to solve for there. There’s also the complexity of a number of objects placed in unpredictable proximity on the oblong coil and it seems understandable that this is a tricky problem to solve.

Add to that:

The AirPower charger is also more advanced than the current competition because it includes a custom Apple chip running a stripped down version of the iOS mobile operating system to conduct on-device power management and pairing with devices. Apple engineers have also been working to squash bugs related to the on-board firmware, according to the people familiar.

This is a complex piece of engineering.

UPDATE: Interesting tweet from Jeff Guilfoyle, with a picture of overlapping coils. The idea being the controlling circuitry would switch between coils as needed. Interesting.