Wishful thinking shouldn't bring back your loved ones even in a fantasy story. This is a time travelling story which has absolutely no rules to the time travelling and breaks the very few ones it has when they find it convenient. The maid being left in the future is ok, while changing any other aspect affects the course of history? Such a proficient maid, with so much action in the story, would certainly affect the course of the story.
You never truly know the device or the set of factors that make them travel through time. At times, it seems like it's the watch, or the magical lady, or the water, or the tree. In the end, it was his wishful thinking. It doesn't make sense.
It also doesn't make sense that everyone has their corresponding person in the future, except for the FL and the maid. And the only person important to the action who isn't in Joseon is the ML's secretary.
They also couldn't decide if the ML was a completely different person or a reincarnation of her Joseon husband. If he truly was a reincarnation of her husband, like it's suggested several times, her not loving her original husband once she got back doesn't make any sense, since she loved him from the start.

I don't mind the evil grandpa, it was a good twist, but, again, it lacks consistency. If the stepmother wasn't evil and was trying to protect the ML in the present, that should have been mirrored in the past, since they repeated countless times that the past was repeating itself.

It could have been a better drama if they had dealt with the inconsistencies.