The similarities between Love Hina and Nisekoi are all there for anyone to see.
Naru and Chitoge are both violent tsundere, much stronger than their MCs, that can send him flying with single punches. Even several points in their graphic designs are similar.
In LH we had Motoko, in NK we have Tsugumi: both warriors and martial, serious, in love with the MC despite despising him for his weakness, them being tsunderes as well.
In NK we have Onodera, which is the fusion of two LH characters, the sweek and innocent Shinobu as character and the role of Mutsumi as the most obvious choice as the person of the promise.
In LH we have Kitsune, in NK we have Maiko: both the witful rascals of the party.
The childhood promise is almost the same as plot mechanism, as well as the uncertainty of the identity of the actual girl the MC made the promise to. Also, all the potential candidates converge on the same spot, together with those who wouldnt stand a chance because are not part of the childhood past but harbourr feelings for the MC engrossing his harem.
Then yes of course exagerations in LH were off the charts, while NK keeps lower even though it cannot be call realistical (people sent flying with punches, walls breached by kicks, strenght reducing drugs, etc. just top of my mind), and yes Raku has a more balanced relationship with Chitoge than Keitaro and naru, and has so many redeeming points more than Keitaro which was a hopeless dolt and a tool, the inn in LH was very central plotwise with all that concerns living together and having to act as the inn's manager, as well as the outside bath was. but those central points are really in the open, no one can deny those.
Maybe the smiliraties are so obvious to us because im-in-love-my-childhood-promise-but-i-be-damned-if-i-remember-who-the-fuck-she-was harem mangas arent tha many yet, but perhaps in the future there will be a zillion more and we will just consider all these elements as tropes instead "it's just a tad short of plagiarism" elements.
All this by saying that i get the point of the OP totally, and that im sick and tired of all these fillers (if they dont have Tsugumi as the main character, as i did with Motoko in LH, those i enjoy)