The reason that you are experiencing the errors is due to the fact that ArcGIS Portal Writer does not handle Aggregates geometry, as the FeatureService needs to be only a single geometry feature see documentation here
If you look under Geometry Type Aggregate.
To be clear, FME can only support Homogeneous aggregates so any input features with fme_aggregate can’t have fme_collection as a fme_type.
WORKAROUND:
The workaround is to convert those geometries into Homogenous aggregate using Deaggregator and Aggregator (in succession). See the sample workspace attached.