You should focus on factors related to your employee's job and performance, as well as how the requested FWA may affect the business.
The Tripartite Guidelines provide examples of reasonable business grounds for rejecting employees' FWA requests. For example, the FWA may lead to significant increase in cost burden, or it may affect your company's ability to meet customer needs. The list of examples is non-exhaustive, and you may include other reasons specific to your company or sector.