However, what's a bigger issue is that they have given it to only half our faction. It's never going to work as long as starborne & coalesced have the same point-values & stats while one can summon and the other can't. And that's ignoring the various other issues already mentioned.
At bare minimum they need to finally make the decision if we're a summoning based faction or not. Cuz this weird middle-ground just doesn't work designwise.
I think this is one of the main contributing factors to why Seraphon hasn't been a very well internally balanced army so far. You have two armies in the same book. The only other book that has truly separate armies rather than just different subfactions is Orruk Warclans. And that book suffers from the same problem. Ironjawz are extremely strong, Kruleboyz are good, and Bonesplitterz are bad. Other "multiple armies" battletomes just treat the different parts of the army like subfactions. The biggest different between us and Orruks is that the three Orruk armies actually use different units, while Starborne and Coalesced share the same ones with the same cost.
So in trying to use the same costs for everything, some units get unfairly shafted because of how strong they are in one army, but this makes them massively weaker in the other. Salamanders are the main example of this. Teleporting or deepstriking a unit of two directly into guaranteed shooting range at bravery 10 is a lot more powerful than moving up the board normally at bravery 5. And likewise, a Bastiladon that can shoot twice for a CP, has -1 to damage and +2 wounds on top of a 1+ save in Thunder Lizards is just way better than any other subfaction. So the units that get spammed in the list where they're the strongest also get nerfed the hardest. But while those nerfs may be reasonably in the strongest subfaction for that unit, it makes them nearly unusable in the other.
And like you said, the biggest thing holding the army back is that GW can't decide if we are a summoning faction or not. They already retconned the "summoned memories of the Slann" thing, so even the Starborne are still living, flesh and blood creatures. Ideally, they would truly split the two different armies into actual different armies and treat them like Orruks. Starborne get their teleporting and summoning, but the mechanic is improved and points increased over Coalesced for summonable units. Coalesced gets better combat abilities like a ward save instead of -1 damage and mortal wounds on hits.
Or, they could remove summoning and teleporting entirely and combine the two factions into one cohesive army. No more summoning, a Slann spell that teleports instead of it being an allegiance ability, and make most of the Coalesced allegiance abilities apply to the new united army. I'm thinking allegiance abilities would be the Asterisms, Scaly Skin as a ward save, and a new reinforcement rule that lets you summon back slain units at half strength from the Realmshaper Engine.
Then for subfactions, which now only give one special bonus, you could have:
Fangs of Sotek: All Skink units get a 6" pregame move.
Dracothion's Tail: Half your units in reserve, but can drop down without a Slann.
Koatl's Claw: All Saurus get +1 rend on all attacks, including mounts.
Thunder Lizards: Stegadons and Bastiladons are battleline. Unique Monstrous Rampage or remove rule of one on monstrous rampages.
And then since most singular factions have at least six subfactions you could add two more Constellatons. I personally think it would be cool to have one that makes Kroxigors battleline and gives them +1 to wound or saves from being near Skinks in addition to +1 to hit. Another one I think would be fun would be one that focuses on flyers. Makes Terradons and Ripperdactyls battleline and increases the range at which the Chief command abilities can be used. (Yes, I know Rippers and Terradons are bad right now, I'm assuming that we're getting major warscroll updates that make most of our army at least useable).
