KG pipes for rainwater, shortage?

Defect? It depends on what is stated in the specifications.
Simply using kg from the ground is actually the very last option, but not uncommon.
It's basically the budget version.
The pipe fades over time and doesn’t look really good.
Something better would be a cast iron standpipe, even better one with a cleaning piece.

You can in my opinion save yourself casting if you're not exactly at risk of crashing into it with a car. KG is subterranean too, when the area is finished you shouldn't see any of it anymore, it will then be cut back during landscaping.