I agree with most of your takes except these two.Over-valued
Haughty Djinn - I just don't think this has the staying power in blue's always-competitive slice of cube. The extra pip needs to represent significant power, and I don't think it does here, archetype support be damned.
Under-valued
Sheoldred, the Apocalypse - Ditto here, you don't need draw 7s to realize the power represented here. Unreal.
Haughty Djinn– If anything, I think this card is being underrated by the community. While I would agree this isn't the most powerful cheap Baneslayer we've seen lately, it is one of only a select few three-mana Blue creatures that can really bring the fight to the opponent.
Blue does not have very many cheap finishers for tempo-oriented decks. Even though URx tempo is a fun and powerful strategy, the playable Blue cards often end up being support for Red prowess dudes. You might get one Murktide Regent as a late-game finisher, but that's about it. Compare this to Constructed where Delver of Secrets is a premier threat that arguably created the contemporary notion of "blue red tempo." In those formats, Blue gets to have both a great early game threat coupled with it's full support package. However, Delver doesn't translate well to most Cubes, so we find ourselves back in the original issue. Haughty Djinn solves this issue by being an early-game scaling threat that also acts as an amazing late-game damage source. It gives tempo decks a nice threat to protect and murder the opponent with. It certainly stands head and shoulder above most other blue threes, which are either painfully small or are highly parasitic to specific archetypes. Really the only blue three-drop tempo creature that seems better than Haughty Djinn right now is Brazen Borrower, and that is mostly because it is a tempo spell stapled to an evasive idiot. It's definitely not played as a finisher.
Hot take: if I were to criticize an overrated blue three from Dominaria United, I would pick the Horizons set uncommon masquerading as a Vintage Cube card. I bet at least a 3rd of the people testing this will cut it in the future.
Sheoldred, the Apocalypse– I think this card, by contrast, is being overrated because of it's showing in standard. Mono-Black is the best deck in standard right now, and Shelodred is missed by the majority of highly-played removal spells. Because of this, people almost always see her when she sits on the battlefield for 3+ turns and kills you, but almost never when she just dies within a turn of being played and effectively acts as a fancy paperweight. I think outside of low-removal environments, Shelodred is going to die before her full value is unleashed.
Sheoldred, the Apocalypse going to be fine in MTGO knockoffs, but everywhere else she's probably just worse than Siege Rhino. Why mess with perfection?
I think this is a fun concept for a post, so I'm going to do a couple of my own. : )
Over-Valued
Shivan Devastator– I understand why so many people are excited to try this card and want it to be good. Big scalable awesome Timmy dragon? Cool! But I think outside of Cubes designed to accommodate big stuff like this, it's probably going to fall flat. It doesn't scale all that well until we get to the 5+ cost range, at which point the mana investment starts to become a liability. I think some people around here who have designed environments specifically to accommodate stuff like this (Cough * @safra * Cough) will have good luck with it, but I think about 15% of the 20% testing this will probably axe it at some point.
Phoenix Chick- This card is fine but nothing special. It's seemed a little bit clunky to me so far, and it's made me not want to play it anymore. I think many others will eventually feel the same way.
Also, the art is not nearly as good as what I was hoping for when this card was leaked. I'm sad about that.
Cult Conscript– Blaggro seems increasingly weak with time and I would not be surprised if people start cutting it. Black has gotten a ton of pushed midrange, reanimator, and graveyard stuff over the past year, and at this point I think the easiest place to cut from in order to accommodate these cool new cards is slots dedicated to blaggro. Black can still serve as a great secondary color for aggro decks without needing 87 1-drops, anyway.
While I think this card will remain popular in Cubes with Blaggro, I don't think 43% of people will be playing it 2 years from now.
Under-Valued
Archangel of Wrath and Urborg Lhurgoyf– My judgment on these two may be clouded by how cool I think these are, but I think their flexibility and brew-ability make them significantly more appealing than other portions of the community is seeing as of now. I bet one of them could see play if Standard stops being mono-black oriented at some point in the near future.
Nishoba Brawler– I think this card will go up with Leyline Binding. It seems very powerful for Zoo Cubes.
Balmor, Battlemage Captain– This is the single best Prowess signpost we have I feel. Most Cubes with redundancy for Red Burn and Blue Cantrips should consider playing this.