Palynomorphs can not be used directly for oil-source rock correlation in sedimentary basins because palynomorphs are fossil pollen than can be recognised and identified in argillaceous source rocks, but they are degraded to various types of kerogen and hydrocarbons during naphtogenesis and do not pass directly into the oil in any recognisable form.

Biomarkers, on the other hand, are specific molecules that can be extracted from both source rock samples and oils, so that a source-to-oil correlation can be established.

You can use palinomorphs, if present in the rocks, to date the rock, to characterize somehow the depositional environment of the source rock,  and to correlate one source rock with another source rock in a nearby basin or sub-basin.

Yes as palynomorphs faced some physiochemical changes during migration and even their size is also lager in some cases than reservoir pores. Thus biomarkers usage will be more genuine for oil source rock correlation.