We investigated epitaxial h-BN and r-BN films grown in CVD using TMB and TEB as the boron precursors and NH3 as the nitrogen precursor. From HRTEM and EELS profile analysis, we find that surface carbon, originating from TMB and TEB, deteriorates the epitaxial BN growth conditions. We suggest that a low level of surface carbon leads to a polytype transition from h-BN to r-BN, while a high level of surface carbon poisons the growth surface, resulting in the growth of turbostratic BN or a layer of amorphous carbon. Our results question the use of organoboranes as a CVD precursor for epitaxial BN and the possibility of growing h-BN thicker than a few basal planes on graphene.