OK so my view might be slightly different to some here. I thought the ending was perfect. I had a baby this year and it really struck for me as showing how exhausting, overwhelming and ultimately lonely in a crowded room motherhood can be. I think that more than explains why Alison wasn't quite herself. The ghosts all giving their thoughts on how to raise, how to settle, how to wind, whatever the baby and waking her up from naps made me feel so sad and frustrated for Alison (and Mike to some extent). Didn't love the overbearing mother in law trope but you know what, that's pretty realistic too.
I missed most of the ending as I was bawling from them getting in the car to drive away. Was there some significance to the room they stayed in?