Okay, two issues so far.
First was mod based. I could ask the stewards to replace a room with a children's room, but the house wouldn't update. I could pay for it repeatedly, but no change.
Turned out to be incompatible with the House Map Marker mod I was using. However, I couldn't actually see the cost of the upgrade before and after changing my mods. It was displayed as "[...] Gold."
The other one was a pathing issue. I adopted Lucia, and soon played hide and seek with her. Her first hiding place was right by the front door in Breezehome (...yeah, good spot, kid), but her second was clipping under some massive rocks.
Luckily she popped back out after I ran around those rocks a couple of times.
So, yeah, I'm liking it so far. Lucia tells me that Lydia said she'll teach her to bake. How Lydia's going to do that with a stew pot is beyond me, but hey, what do I know?
EDIT
Can't seem to dismiss my follower Rayya. She has the dialogue and everything, but keeps following me, no matter how many times I tell her to go away.
And Ria, my steward, asks if I'm lost whenever I enter the house.