Note: Credit to @Zestanor and @Truffles for this review.

Since School Daze, the Young Six have been some of the best characters. Eccentric and childlike, yet diverse in personality, gender, and race, their friendship is linked by caring for each other. After Cozy thrust doubts subconsciously, the Tree's spirit reminds them of their powerful friendship. Uprooted is their second (unplanned) test: The Tree's detroyed to their massive shock, and they concluded to memorialize it.

So how well was is written? Not all that bad.

In only a couple of minutes, the opener effectively establishes what their personalities, goals, and weaknesses are (often intertwining them through jokes, whether they’re brand new or called back from previous episodes, e.g., Smolder’s closeted femininity). Then when Twilight tells them the news of the Tree’s destruction, it’s easy to see why they’re devastated. The Tree not only solidified the harmony between them, but Equestria’s as a whole. Additionally, Dubuc addresses to continuity from What Lies Beneath and School Raze while keeping it self-contained so newcomers mustn’t watch earlier episodes to understand.

At the same time, it gets clumsy at points.

1. Their commemoration for the Tree sometimes gets repetitive, referring to it by name quite often. Altogether, dialogue's serviceable.
2. Sometimes it can get quite preachy, treating the Tree, the Elements, and their messages of friendship as religious relics. Granted, that’s the point. No one can agree on how to honor it without desecrating its legacy, and their solutions range from complacent (the statue) to one-dimensionalizing (the friendship forum) to selling out (Gallus lying about the Tree to make the cave a tourist attraction). But personally, they can make the subtext a little bit more subtle.
3. Gallus’s money-making scheme is cringeworthy. I laughed at Yona’s reaction, but not his embellished plan. Dude, you ain't no Flim and Flam!
4. Sandbar revealing to stashing the Tree's broken parts in a wagon out in the open is the episode's worst moment. The whole dilemma throughout Act 2 is bound by Sandbar's decision to clean up the cave in favor of his little plant, and there's no second exit. For Yona to find it really easily (especially after Gallus asked him where it is beforehand) and nearby one of Silverstream's murals makes the remaining five look incompetent throughout their argument.

However, Uprooted has several bright spots.

1. The whole montage is fantastically set up and executed.
   
   Early, the students departed for the School earlier than what their families and tribe leaders permitted. Unlike the others, Thorax balances out severity with worry and understanding the most naturally. The Changeling Kingdom altogether's one big family and successfully adapted to his fight-less vision, and the last thing his kingdom wants to do is get into another war, which almost happened last season. However, he'll agree to a suggestion beneficial for everyone.
   
   Thorax and Twilight agree to the Y6's plan, leading to one of their best montages (and to echo @Zestanor, "one of the more significant parts"). In addition to establishing a very strong reason why everycreature returned to school so soon, their cultures are presented authentically. Some of it, like Smolder winning an arm wrestling match, is silly (and Gallus's was dark humor), but none are shown to shoot them or their homelands down. They all back up their personalities, subtly expand their lore, and affirms their care for each other and their homes. My favorite's Yona: If you didn't awe from clearing the snow for her dad and this, then y'have no soul.
2. Speaking of Yona, she's fantastic, as usual. This time in a voice-of-reason role. When everyone's superficial tributes didn't capture the Tree's heart, Yona stayed on the sidelines, waiting for everyone to listen to each other, only to finally speak up and remind them what the Magic of Friendship. Sure, the Tree may be gone, but not its memory.
   
   BTW, her explanation for the yak's love for smashing cleverly develops more into her culture and really shows there's more into their lives than just aggression. Not a bad evolution after Party Popped wrote them as savage and primitive.
3. Spike and Twilight are written well, retaining their complementary relationship while helping the Y6.
4. This episode climaxes with The Place Where We Belong, S9's and the Y6's first song. Sentimental yet hopeful, its tone and message are wonderful. While things and lives come and go, their memories don't as long as we remember them and use them to self-improve.

It feels a lot like an allegory about death, but rather than replace it for hibernation, there's a true sense of finality with Sombra destroying the Tree and Elements and the unknown of how to properly tribute it. Their decision to build a small treehouse solves that problem wonderfully. Yes, it's not the best built, using broken trunks, branches, and spare parts, but it centralizes who the Young 6 are: a diverse cast who represents the best of each kingdom and each other. The following lines punctuate it:

Quote

Sandbar: Well? What do you think?

Ocellus: It's, uh…kinda messy.

Smolder: Yeah…like a bunch of different parts all smushed together.

Silverstream: Oh, it's just like us!

Yona: *rolls blueprint* Yona think it perfect!

Gallus: Let's give it a try.

But then, a miracle. By representing the Magic of Friendship at its purest, they rebuilt harmony within each other and applied their close friendship to rebuild it in its memory. Consquently, the Tree of Harmony turned to this:

The Treehouse of Harmony's beautiful! Its crystalline pastel colors of blue, pink, and yellow invite the eyes, and it's breathtakingly composed. Compared to the Castle of Friendship, you'd want to actually go there. The Tree's spirit says they'll be safe within the house's walls, and optically, you believe her. Also, the ruins of Castle of the Two Sisters is collapsing (credit to @Truffles for pointing this out), so the Treehouse now takes its rightful place, replacing it for something more useful.

As far as the elephant of the room's concerned, sure, the inspirations to Castle Sweet Castle are obvious, thanks to its similar structure and ideas, but the plots aren't the same. In fact, there's one gigantic difference between them:

1. In CSC, the RM5 want to make Twilight's castle feel more like home. Twilight stayed away from it because she found it to not feel as homey as the Golden Oak Library, and no one found a cohesive solution for vral hours. So what do thy do to? Use the Golden Oak Library's roots as a tribute to her old home to connect her old home with her new one.
2. Here, the episode's about memorializing the Tree only, but none of the Y6 could come up with a proper solution without desecrating its legacy. Until Yona reminded them of how they became friends and became closer as a result of the Tree's test. So they used the Tree's old branches, trunk, and Elements to build a treehouse, and the Tree used its powers to create a temple.

Coming into Uprooted, I predicted the Y6 will become the new Bearers of Harmony, for the Tree tested them, saved them, each on sharing similar qualities with the ReMane Six (while still being themselves), and the Tree encasing them with the Elements' glows encased them in Raze. But with the auras being trading interchangeably (compare this to this), the Elements and Tree evolve into the Treehouse, and the spirit wants the Magic of Friendship wanting a safe space for friendship, it's becoming more and more possible that the Tree's spirit and magic used when the Pillars planted the seed want to pass down the MoF, perhaps to everyone, not just six masters. The fact that we also see no cutie marks may foreshadow more of what's to come.

Altogether, a really good episode.