The mechanics shown off fit very well with other existing factions & to an extend are contradictory (why are they combining light-symbolism with earth/mountain-symbolism of all things...). From what's shown it kind of feels as if they've collected all the cool ideas they've had over the last few battletomes and saved them up for these. This is especially noticeable as several of these would fit extremely well with some of the tomes that have recently released (us, ogers & fyreslayers most noticeably)
Shining company is a great example of something that'd fit well with seraphon as we're made out of stardust and literally bleed starlight. It also fits well with SCE as Sigmar's angels who descend upon lightning. The disciplined nature also fits well with both us and the SCE. But for some reason it's given to elves. In general, most of their whole light theme fits with both us and SCE. The only bit that seems to be somewhat uniquely lumineth is their focus on speed & fluidity where we combine it with a primal savagery and the SCE combine it with a more solid formation (though the eart/rock aspect of the lumineth encrouges on the SCE there as well...)
Having them all be wizards makes them the sacrosant chamber on steriods.
Also generally speaking the magic we've seen from them, especially in the leaked battlereport, puts our spell-lores to shame & their guaranteed casts and modifiers put both us and Tzeentch to shame.
Enduring as a rock is weirdly out of place, a faction that focusses on fluidity & light-symbolism suddenly takes on aspects of heavy earth. There's any number of factions on whom this'd fit better. Fyreslayers being the most obvious with their connection with mountainholds, lava etc.
Tectonic force is similarly out of place and would fit much better with a destruction faction like ogers or orruks, or even on fyreslayers.
aetherquartz seem to be a light variant of the bonereapers' command abilities that totally aren't command abilities.
Also, it's yet another super organised army fighting in perfect unison on a (near) subconscious level, much like us, the bonereapers & the SCE.
And that's just the thematic stuff that feels like should be part of other factions. We haven't even gone into how some of that stuff just looks stupidly powerfull and can make you feel cheated for that
Giving your entire army, or at least the entire battleline, mortal wounds on a buffeable 6+ to hit does not seem like a "fine balance". That sort of nonsense needs an absolutly ridiculous drawback to be anywhere near balanced.