I'd argue that for a fast food chicken sandwich (especially the "spicy deluxe" variant), theirs is pretty good, but these are matters of taste.
If I go to a CFA, I can count on the food being a certain, consistent quality. I have eaten a lot of CFA over the years, and I've had a low-quality piece of chicken (e.g. tough, full of gristle, etc.) maybe once. Compare this with "chicken" served at milkshake and burger fast food joints, which is mostly made from parts and not whole meat. I've gotten violently ill from KFC before and have since desisted from buying their food, and my local Popeyes has such bad service that no one goes - I think DoorDash is singlehandedly keeping them in business. Even Bojangles in the South or Hardee's/Carl's Jr. have given me real clunkers of sandwiches and chicken before. Chick-fil-A is consistently good quality in comparison. When I'm done with a meal at CFA, I never feel like I'm going to spend the next few hours wondering I need to begin praying to the porcelain gods.
The other killer feature is the service and efficiency. I don't hesitate to get in a long CFA line, because I know it will clear soon. This is another aspect of consistency that is just underrated. If I get in a long line at, say, McDonald's, it may clear one 5 mins or 30. With CFA, I know it'll very likely be done in 10 mins.
The third really big differentiating factor for me is that when you get something you expect to be fresh from CFA, it is consistently fresh. I have not experienced wilting lettuce, "gelling" tomatoes or carrots that have been left too long at CFA. At any other fast food joint, that is a regular experience, even the ones that are too posh to have a drive-thru. Their fresh fruit is the same thing - consistently high quality and tasty. Same goes for lemonade and iced tea (theirs really is the best in the business; and for those not from the South, you can always ask for diet or unsweetened versions of fresh drinks). My son actually gets upset at me when I accidentally get him a side of waffle fries from CFA rather than the fruit cup, since the quality is so good.
The last part is that they seem to really treat their employees well. CFA tends to start their employees at higher wages than almost any other fast food chain, and they give them extensive training in how to operate the store and treat customers. I've heard employees - both Christian and not - say that having a guaranteed day off once a week is wonderful. The restaurants are clean and the workers seem to take pride in doing good work. It's just a good atmosphere. It seems like the kind of job that gives low-skill people the actual skills they need to be productive citizens and employees when they're looking for something beyond food service.
If I were to put it all in statistical terms, along almost any measure dimension, CFA has an above-average mean and extremely low variance. Other fast food places have optimized certain dimensions like this as well: McDonald's for example has extremely consistent - and mediocre - quality and a very consistent low price point. None of them match the breadth of consistency as well as CFA does.