FA's are only required to give out our first names. Any letter to WHQ w/ flt # and date with brief details ie FA working C cabin( if you didn't get their name) and you can be sure that the FA's supervisor will hear about it from WHQ. and call the FA in question in on the matter to get their side and write a report back for WHQ files.
WHQ is 90% of the time on the pax side of the issue unless your letter is so far out there that they know it was written to gain some type of free upgrade due to some outrages story.
I know first hand since I have been called in once over a letter on another FA working the flt Y cabin. It was one of the first times I was happy that I chose to work in F during a flt.

UA does keep track of pax that write in a lot over things just to get upgrades or miles. They have gone as far to ask the pax to not fly on UA since they can't meet their expectations.