I've trying use spatial anchors for my Hololens app, that persists holograms across app sessions. My app does't run on android or ios: runs only on Hololens, also does not share anchors among other devices.
I think that it is enough to save Unity World Anchor to local storage for this usage.
But, I only concern about locating accuracy. Does differ locating accuracy between Azure Spatial Anchors and Unity World Anchor?