Installing 15" deep wall cabinets on a 14" deep soffit
If the cabinets are deeper than the soffit, you have a "shelf" above the cabinets then. That would collect dust and cooking grime and be a pain to clean IMHO, let alone promote a spot for spider webs and the like. I would add a couple of sheets of Sheetrock on top the soffit wall, tape and cover it and make it all flush.