I recently played through the Fallen Spire campaign to the end. It was on a 50-planet X map, with 2 level 7 AIs (Vanilla & the one that uses Counter Spies). All in all, it was really good fun, the Spire starships are fantastic, especially the higher-level ones. I have to concur with the feedback about it being a bit 'grindy' as it goes on, and in some ways perhaps it was a little easy. However, I think some of this was down to the nature of the X map and the way I was playing it; choosing a home planet with only one wormhole, clearing out each branch entirely to make defence easier, and having an incredibly well defended central planet (funnily enough, called Murdoch) with a hub on it, and a Superfortress, and an Orbital Mass Driver, and a Radar Jammer mk II, and a Ion Cannon mk IV. Perhaps I'm ready to move up a level with the AI difficulty, but I was recently thrashed with a CPA on a regular style map, so I thought I'd hang back for a bit.
In the end, I'd captured every planet except for the two home worlds, and put hubs on all 5 'central' planets (you'll know what I mean if you've played an X map) covering the wormhole back towards the home planet. Capturing some of the later shards was pretty intense, in a good way though, and the AI did make some limited inroads at times. After getting 3 of the hubs it would have been probably reasonably easy to take the homeworlds, but I was curious to see how the Spire campaign played out. I do wonder though what this would be like on an 80-planet map, where the capture everything strategy is less workable due to AIP.
Maybe make it so that the Galactic Capitol is buildable with a lower number of hubs, and the shards beyond that give something other than more Spire colonly ships; e.g. special ships or defensive structures, extra unlocks for the Spire starships, or allowing the hubs to be upgraded an extra level with more shipyards, habs & reactors. This could cut down some of the grind, but still allow access to some of the advanced Spire stuff. You could introduce a shortcut to the end of the campaign (but only after a certain milestone), so there would be the option of getting further normal shards, or go straight for the final shard that brings in the Spire fleet with all the extra risks that brings.
I loved the idea of using the standard unlocks for things like laser turrets and beam turrets on the Spire Destroyers and above. That really opened up a lot more options, and it was good to unlock and use some things that I'd normally not bother with, mainly the higher-level beam turrets that just obliterate AI fleets, provided that they don't get overwhelmed. The Spire ships were really good fun and worth the effort and time to get them, and in some ways a lot of getting the later shards was because I wanted to see how the story panned out.
The campaign isn't at all 'broken' in any sense as far as I can tell, but could possibly do with a few nudges in certain directions.