What's Shounen Jump Syndrome, you ask? There are actually three types of this: (1) is the type that gets low ratings and gets axed because it was just the same thing as the more popular ones but worse, even though the one shot wasn't; (2) are those that get low ratings despite interesting and different content from the big three and consequently rot and morph into the same kind as the popular series to become more popular; and (3) are those that start off interesting then get high ratings but becomes a victim of its own success as the magazine will try to suck the mangaka dry to make it as long as possible and rake in as much money as they can regardless if the story is still interesting or not.
Nisekoi is the third type.
Yes, I just invented this on the spot in jest, but Nisekoi is actually suffering from being dragged on for too long due to its popularity. The story is good for about half the chapters it has now (as of writing, there are 96); the mangaka probably expected to be axed again early on and was unprepared to make a serial this long so I guess we can't really blame that on him, but still, it's suffering from its longevity.
It started off good, and the love triangle was actually intriguing. Who did he make that promise with? What will he do when he finds out if one or the other is the one? Will he break up with his fake girlfriend and potentially start a gang war if his crush was the one, or will he try to make their relationship real and give up on his crush if his fake girlfriend was the one? - Those questions will keep bugging you and you will be left begging for an answer after each chapter.
But then another girl came. And then another. And then the story goes in circles, having random episodes that has no real significance to the premise. Although, there are some that do hold some weight for some backgrounds and character development, but will all of them be important to the conclusion of the premise?
Now, Love Hina, which Nisekoi has a similar premise to and is also a harem series, had more than 100 chapters, why can't Nisekoi can't? Because the promise in Love Hina had a prerequisite to get into a certain university, which its main protagonist strived to, and failed, and strived again; the girls were already there at the beginning and grew attached to the main character. Nisekoi promised marriage when they meet later on in life but the main protagonist only forgot who he made that promise with - that in itself is not a problem if it was being tackled this long, but Nisekoi's longevity is due to other girls popping up out of thin air, making it feel like the mangaka is stalling the story by adding more content outside the main premise.