It's not too late to write that letter.
I would start it out like this:
We have been planning another trip to Orlando and have been considering our activities for our upcoming stay. We are likely not going to make Universal part of our visit because during our last trip.....
I had a terrible experience at Universal Studios, before Islands of Adventure, with a security guard named Paul who was so rude to me when I was standing under some shade at the Twister attraction. My family went ahead of me into the long line that was mostly out in the sun. You see, I am allergic to the sun and I break out into hives within ten minutes of standing in one place, so my family was going to walk the line, then I would join them when they were under the cover, just before the entrance. "Paul" said that if I butted in line, he would arrest me and throw me out of the park. He was completely rude, though I explained my allergy and was polite to him. I joined my family in the sun because Paul was such a bully and I was by myself, so I felt helpless. He ruined our only day in Universal, 1/7 of our vacation days which are such precious times as a family (they are all grown now and were in their late teens to early twenties in 1998, when this happened). To make a long story short, my husband insisted on stopping at Guest Relations on the way out. They called Paul into the office, he was forced to apologize for belittling me, then they gave us all two-day passes that would be good forever and reimbursed us for that day's passes.
What really hits the ball out of the park is when you tell them that you have never had anything like this happen to you at Disneyworld. They just hate to be compared to anything Disney. They strive to be as good as Disney but always come up short, in my opinion. Their numbers do not lie, and their numbers are always less than Disney. We visit Disney three to four weeks a year, just the two of us, and we spend one or two days a week at Universal and the rest at Disney. It's the Disney attitude that keeps us coming back. Universal has great rides, two of their rides are our absolute favorites, but they lack that special something that will always keep them second. The company wants that loyalty from its customers, but issues like you had will never get them where they want to be.