Boy, do I have a lot of feelings about this DLC. It feels like it wasn't thought out at all.
My first issue is the layout of the areas in which you're able to build. Due to the fact that there's no terraforming in this game, you're stuck with the uneven and narrow paths connecting the three sectors you're able to build your vault in. Combining this with the limited prefabs you're able to build with, there's certain areas that you're not able to neatly connect to at all, meaning that if you want to utilize all sectors you'll just have to use the natural tunnels instead of having a succinct vault. It's incredibly frustrating when you have a vision for your vault, only to realize that what you want to do isn't possible because of the limited prefab options and preset terrain.
The next biggest issue for me is the lighting. You can build a light on every single ceiling and wall tile, and there will still be huge patches of shadows that you can't get rid of unless you having a ceiling made of light bulbs. It's going to look tacky either way.
My third gripe is a bit of a smaller one, but I think there should have been a tutorial for the way power works in your vault. The game pretty much hides the introduction of the Vault Power Conduit, which is used to externalize the power that is already being carried through your vaults walls. I had so many wires and conduits strung up throughout my vault before deciding to look it up online, and found that many others also didn't realize that the vault power worked like that.
Part of me is excited to try and build a vault again every time I come back to this game, only to always rediscover that this DLC is, above all else, frustrating. The ability to build your own vault would have been a great idea if it was well executed, but there's just too many limiting factors for me to actually enjoy it.