The both have a spooky-like style and involves supernatural creatures (Hotel Del Luna involves ghost and the afterlife world while Tail of the Nine-Tailed deals more with mythological creatures such as the foxes, etc.).
The male lead of Tale of the Nine-Tailed and the FL of Hotel Del Lune both are more than 1000 years old and have a past that ties with someone important that they have lost.
Also the concepts of reincarnation and past lifetimes are an overarching theme in both shows.

Same author, although the story line of Love of Thousand Years deviated more with somewhat questionable execution for LoTY. Fantasy elements. Involves lifetimes and the male lead sacrificing for the female lead. Great acting from the male leads of both shows.