Use ～上に in reports, proposals, and written explanations to stack a second fact onto the first when both point in the same direction — two merits or two drawbacks. It is noticeably more formal than ～し or ～だけでなく, and carries an evaluative tone, so it fits summarizing why a plan is attractive or why a situation is problematic. Avoid it when the two clauses conflict in polarity, and don't use it before requests or invitations.