Can cyusb3.sys handle composite devices?
No, cyusb3.sys is a generic vendor-class USB device driver that assumes a device has only vendor interfaces. Composite devices are bound to Windows composite driver (usbccgp.sys) by default. This exposes each interface (or set of interfaces based on the Interface Association Descriptor) as if it were a separate USB device. Devices containing vendor interface(s) can be bound to cyusb3.sys.
The composite device part of the overhead is handled by usbccgp.sys.