The problem with shipping a niche ship: you read all the fanfiction in one afternoon and if you want some more you have to write it yourself.
The problem with shipping a popular ship: 16,835 results on AO3. You start playing with tags and sorting through it, full of determination, confident that with so many fanfics you’re bound to find something you’ll like. Two hours, 30 instances of awful writing, 8 squick-outs,13 wtf AUs and 157 just plain uninteresting later you have to rush back to the canon to even remember why you liked the ship in the first place.
The problem with a popular ship: there are 6,285 fics similar to yours and you can’t help but compare your writing with others
The problem with writing a niche ship: you write only for yourself + your best friend who’s either your enabler or just feels sorry for you