The GeometryReplacer accepts GeoJSON as an input format but will fail if _full _ compliant GeoJSON is supplied. Instead, the GeometryReplacer expects just the geometry component of the GeoJSON, which means this needs to be extracted from the JSON somehow prior to being passed into the GeometryReplacer.
It would be great if the GeometryReplacer could be enhanced to accept any valid GeoJSON.
If the GeoJSON is a multi-feature object, then the GeometryReplacer could either create aggregate geometry, or provide an option to create multiple FME features.
There are several questions in the FME Community about this. Here's an example: