I am quite disappointed, not necessarily that we don't know who the watcher is, but because they tried doing everything they could to make it interesting, twisty and mysterious that it turned out to be a whole lot of nothing.
I thought the ending itself was fine. People had so much trauma/past relationships in this house and kept coming back. Kept watching it. In the end, they're all watchers because they can't let go of their past.
However this doesn't justify who the duck is murdering pets, breaks in, phones them, sends the letters.
Perhaps it's the preservation society William, Jasper and the lady with glasses, the masterminds, they're trying to scare the owners to preserve the houses original state. We'll if it's so then they failed hard.
Overall they tried high quality tricks but too many of them made the whole thing very cheap.