Are there any materials which are NOT suitable
to be processed with a melt pump?

For example, PLA is extremely corrosive, and PVC, in addition, also heat-sensitive. But both materials can be processed with eprotec melt pumps.
In general, if corrosive, heat-sensitive or easily degradable materials are intended to be processed, detailed clarification and communication with the gear pump manufacturer is required. Beside thermoplastic materials, gear pumps are also commonly used with rubber (NBR, SBR) and TPE’s, SEBS and SBS.