Sympathise with that feeling - it happens.
The book's not on general sale in the UK so I have to go by opinion and hearsay, but there is one aspect touched on here that bugs me and that is the use of Umbra. To me the Umbra I met at the edge of the ashlands was an inspiring guy and his sword was a medium kinda artifact - useful to a point but not so special in itself. You could use it to soul trap and you had longer to make your kill, but that was all it showed.
That Morrowind artifact had 120 seconds to make the hit and capture the soul, a limited range and the spell could only target one person/creature at a time. Plus it had to recharge as all atrifacts do. It was an enchanted artifact and so could not be changed. That 'was' the nature of the sword ...
Now it is a mega-fantastic thing that powers a city ... how?
There is precedent for a flying citadel - see the ES game Battlespire ... but that required something rather more than a tiny underpowered artifact like Umbra to fly. I do not underestimate the power of necromancy either - simply the use of Umbra in the way it is described does not ring true to me.
Now all the devs in the world - or just one would be enough to make it cannon. But what has been described here does not sit right.
I would have been better 'Lore' for example if there had been a new artifact linked to the original Umbra in some way perhaps. Are you guys sure you are not leaving out vital detail in your descriptions?