How likely is it just that Huawei devices are the most common that use the usb modeswitch technique? I've gotten 3 different "free" 3g/4g wifi routers (all rebranded to the ISPs name) and they were all Huawei.

ZTE is also extremely popular but it's too short to search for. Other vendors like d-link, tp-link, alcatel, novatel, qualcomm etc all turn up around 300 results each.