Honestly, the events leading into this were great and any harem manga that has an actual ending gets some respect for me so I'm probably being overly harsh. I guess the resolution to the whole necklace mystery left a bad taste in my mouth, Raku should have just done some form of "who cares whats in here, I do what I want" instead of this forced flashback bullshit, it would have made the ending much more interesting and believable, but alas we have to deal with the shounen romcom version of Final Fantasy 8.
Frankly speaking, I always got the impression that the locket mystery was just something Komi had to deal with because it was introduced so early in the story and kept on showing up as the thing to drive the plot whenever needed. You can tell that by the end the locket itself became less and less important the moment Yui got rejected because it wasn't even a factor in Raku's decision, or wasn't mentioned when Marika's arc was playing throughout. Once Chitoge ran away and Raku decided he was in love with her, all it took to unlock it was Yui taking out her key piece.
It was such a throw away thing, that I get the frustration with it being in the plot for as long as it was. But that's what a McGuffin is in the end, something to drive things along when its convenient. I think that Komi didn't fixate on it once Raku got his memory back before openi the locket and just went "oh, so Onodera is my ..." should be the clue that it really was a throwaway plot point that overstayed it welcome. But Komi decided to tackle it head on because he had to, to bring everything to a close properly. I think that final point is what makes me lenient on the whole thing.
I would have much preferred Raku not ending up with any of the key girls, as a sign that he did realise that regardless of whatever promise his 5 year old self made, time changed him into a different person and he just has different interests and feelings, and that he wouldn't be hung up on the past as much as he did.
Though credit is where credit is due, I think that Raku's reaction at the confession and his "no regret" the following chapter did do a good job in showing what was lost (3 years of him and Onodera loving each other, but never confessing, ending up having his feelings change and the both of them never experiencing what their life togther would've been, during that lost time) and also how he was committed to not wallow on the past (because he did change after learning more about Chitoge).
Onodera should have had a more concrete resolution, with her feelings also having changed as well to tie things up, but that's a fault in Komi committing to a pure love triangle and her being the second most bland cast member out of the entire series, after Yui.
Which says a lot given there is a literal Chitoge doppelganger around.