ᐅ KG pipes for rainwater—are they problematic?

Defect? It depends on what is specified in the scope of work.
Simply using a pipe made of ductile iron directly from the ground is really a last resort, but not uncommon.
It is basically a budget version.
The pipe will degrade over time and doesn’t look very good.
A bit better would be a standpipe made of cast iron, and even better one with a cleaning section.

In my opinion, you can skip the concrete casting unless there is a risk of a car driving into it. PVC sewer pipes are also underground. Once the site is finished, you shouldn’t see any of it anymore, as it will be cut back during the landscaping work.