[wlesieutre]

Apple's official note says "You should not connect any power supply that exceeds 100W, as it might damage your Mac." [1]

This is the maximum wattage allowed under the USB Power Delivery spec [2], so any USB-C power supply should work. Maximum voltage/current is negotiated between the supply, cable, and computer, which should select the highest amount supported by all parts of the system.

If you're using a cheap 3rd-party charger that doesn't follow the spec and decides it's going to tell the computer "I support 100W, let's do that!" and then jacks up the voltage beyond the USB power delivery limits, then yeah, expect to have problems.

The problem that I actually imagine happening is people getting a 20W rated cable with their cell phone, trying to power their computer through it, and wondering why the battery continues to drain.

[1] https://support.apple.com/en-us/HT207256

[2] http://www.usb.org/developers/powerdelivery/