There's no strong emotional development between the main leads; the male lead's acting is average; there's no proper explanation about the future (present). If there's time travel, going back to the present is an important element of the story, but without dealing with it, they give a simple ending with the past. 
 
This drama is good for K-drama beginners (time travel genre), but after watching many time travel dramas for me, they presented a simple revenge drama with the time travel element.
 
They had the choice to create a more dynamic relationship between the main leads by using her ex-boyfriend, and they had to create love triangle points too, like one spring night, but did not utilize those elements. It's better to sometimes use a third person to create strong emotional development between the main leads, like One Spring Night. If they use it more, it will become trash.