Ai-chan's problem with fanfictions is that the self-insert characters are always portrayed as being better, smarter, and more capable with plot armour everywhere and that the original cast always look at them with awe and admiration (or antagonistic towards them out of inferiority complex). This Mary Sue trope is a common feature in most fanfics. It is super annoying and the reason why Ai-chan never read fanfics anymore since turning 25.
For example, Ai-chan read one Stargate Atlantis fanfic where a special forces soldier got stranded in the Pegasus galaxy and singlehandedly destroyed hive ships with homemade incendiary grenades. The author justified it that because the insides of the hive ships are squishy, it must be flammable. Ai-chan was like, "No, it's not, dude!" Like, have the author tried to set living tissue of fire? They don't burn because they have a lot of water inside. A single incendiary grenade wouldn't have any impact other than some scorch marks, which would eventually disappear because hive ships are biological and can regenerate their cells.