There's more to it than merely listening in or gathering metadata. In the future we'll all be used to widespread 5G, and increasingly dependent on it - it's already commonly touted as enabling self-driving cars, IoT etc. and even replacing WiFi.

People are concerned that Huawei / the Chinese could effectively shut down important chunks of infrastructure that would cause chaos in a city like London, and many other places, and furtermore that the implied threat of such a mishap, might be used as a form of coercion.