Just because something is complicated (just for the sake of complication), does not make it good!
The basic plot always is - problems of time travel and grandfather paradoxes and time loops, like any other time travel story...and how to stop it.
1) Background score although good, overused, thereby killing its significance. There should be lapses that prepares the viewer for an upcoming suspense and revelation.
2) What is achieved through bringing 4 family trees together ? There could have been 10 families, with more complicated timeline interactions, but that would have not improved or assisted the storyline's motive in any way.
3) The missing kids are given so less screen time, that no one actually empathises with the disappearances. Hence, fails to establish an emotional connection. For that matter, a viewer can never truly form an emotional connection with any character.
4) 3 seasons , 60 minutes runtime with 28 total episodes! Really?