Saw a comment on reddit that aptly summed up why this is so annoying and unsatisfying an ending, why a lot of people, including myself, thought Yotsuba could have been a bait ending, and to paraphrase it:
We essentially never saw or read any of Fuutaro's personal feelings on each of the girls, and were never given a clear indication how he actually feels about each girl. Yea, there were hints and all that for each girl, yes all the girls clearly liked him, but because of the lack of exploration of Fuutaro's feelings, while the winner remained hidden and unexpected, the entirety of the relationships were one-sided and lacking chemistry, thus any choice for any girl would and does feel unsatisfying, forced and like a bit of an asspull because of that.
This manga, like the majority of harems, would have been better off as a VN or pick your adventure book. I have a feeling this will be forgotten just as quickly as Nisekoi because of this incredibly whelming ending.