Disappointing in numerous ways only because it had a lot of solid pieces to it.
The first qualm is the lack of development with their families. Realistically, they should have been a lot more anguished over being torn away from their homes, and this would have been accentuated during their infrequent visits. But the manga wanted to spend its focus elsewhere, and that's actually fine. Second, there probably could've been more focus on the physical/mental discomfort of the change, particularly into the titular adolescence period, but that's a hackneyed trope at this point and honestly can be a bit cringy, so largely skipping that was fine too.
Where the story excelled was in character development, showing them slowly adjust and rebuild their lives under new circumstances. Their personality changes felt natural and true to their cores, refreshingly eschewing the "body affects mind" trope (that can be fun, but can also give a bit of a shlocky feel).
The greatest and fatal failing of the manga was that the romance sucked. Throughout all of these nice character developments, Yuuta and Yui actually grew more distant, to the point that they hardly interacted (outside of story focus) once they were in high school. This was actually interesting and made sense - they'd always have the fundamental connection of the swap, but their interests and personalities were different to the point that growing apart was natural. They remained friends, but there was practically no romantic development aside from Yuuta's one-sided declaration. Then nothing happened for years as Yui and Kazuma had a lot of mutual screentime that progressed their relationship. Really, the momentum of the story was for those two to get together post unswap, and would have brought those two characters to a sensible resolution.
But such an ending would have been far too tragic for Yuuta, the more sympathetic and active of the two protagonists. Especially post unswap, he would've been unable to relate to the friends he'd made, forcibly estranged from his two closest friends in Kazuma and Yui due to the awkwardness of that situation, and set on an academic-type life path he was wholly unsuited for. The only good ending for him was to remain in Yui's body and end up with Tachibana. That would definitely require a whole lot of mental anguish on his part, but slowly accepting his lot in life would've also been thematically and emotionally satisfying. However, this is, of course, mutually incompatible with Kazuma and Yui getting together.
So it makes sense that the author went for the forced relationship between Yui and Yuuta. The characters are compatible so it's easy enough to imagine they could get together. But this really should've been done in the 50+ chapters before the ending, rather than making Yui suddenly attached to Yuuta in a way she'd never before demonstrated so that she'd reject Kazuma. It was perfectly fine to develop the romance between Yui and Kazuma and have this ending, but that romance was to the exclusion of any connection between Yui and Yuuta, rather than in parallel. Heck, even disguise it as protectiveness, something seemingly sibiling-like, or whatever. But the only attachment between them, aside from being normal (not especially close) friends, was their concern for how their (former) lives were being upkept. That is, Yui being romantically interested in Yuuta, especially without any physical basis, was wholly unconvincing. The explanation of how it all began with him being a "hero" for her was nice, but too late to make an impact without the necessary setup.
Despite expending a lot of text detailing my complaints, this was an enjoyable read. Again, seeing the characters grow was worth it despite the missteps and dissatisfactory resolution. It's nice to see the genderbend topic be treated seriously and have a real impact on the characters rather than be discardable clickbait, and, though some of them are undermined by the ending, the substories are still enjoyable.