First, a complaint about a product, a policy, or an employee?? Makes a difference in how to proceed.
If a product or service, I recommend: Product Feedback - Apple
If a policy complaint, I recommend using the main number instead of the Support line, as main board operators usually have more routing options:

IF ABOUT AN EMPLOYEE: In a former career, one of the "hats" I wore was to manage consumer communications for a major company in another sector. That included complaints. For employee complaints, I preferred getting a cogent business letter that unemotionally detailed what the person involved did or did not do. Details help. Date; specific issues; etc.
A letter gave me time to investigate before responding and, whether an "attaboy" or the "other kind," became part of the employee's permanent file. Very useful to have at review time.
One man's opinion based on work experience in such matters.