For those not wanting go search:

Mozilla's Position

Devices that offer serial interfaces often expose powerful, low-level functions over the interface with little or no authentication. Exposing that sort of capability to the web without adequate safeguards presents a significant threat to those devices. A user deliberately installing a site-specific add-on may be adequate, given sufficiently understandable consent copy.

Seems reasonable to me.

Google mainly built this WebSerial shit because they HAVE TO to make Chrome OS more than just a useless web browser.