Firstly I am a massive supporter of the Apple Photos sync, I really think its excellent.
One issue I have come across when I am syncing my wife’s photos, is that it will sync a preview of the photo to the vault but won’t sync the original, most often as my wife won’t leave Mylio open for long enough for this to complete. She will then go through and tidy up her photos and delete some, this leaves a number of previews (stub’s in my world) without any hope for the originals to be synced! But I won’t know this straight away, I will initially think she just hasn’t left Mylio open long enough to sync, until I do that myself and see its finished syncing, and then assume she has deleted the photos, there is no easy way to reconcile that fact.
Not sure if there is an easy way for Mylio to be able to see that they were deleted at source or something like that but it would save a bit of time in this scenario which is quite common in our household with our setup