Birch is likely safer than melamine as it does not contain Formaldehyde in anywhere near the same concentration. Is the ply in question be baltic birch ply? If so it is waterproof so it is very resistant to humidity more so that melamine. not to mention is is way stronger especially the baltic birch. I would suggest it is better than melamine in all respects but price but if your wholesale price looks anything like mine there is no reason to use melamine at all. Perhaps the sealing of the wood is the only possible down side.