The thing that I personally don’t like about Gaiden Chapters is when the condition to them isn’t stated in the game (or is just unclear) and I have to look it up.
If I’m given a text box at the start of battle stating the conditons or something, any complaint I had in regards to them would be rendered null. After all, the side-quest with Nino and Jaffar in FE7, as well as Chapter 5x in Sacred Stones, are two of my favorite chapters ever.