Both are science fiction based movies with Similar powers, and themes as well as special effects. If you like one you'll like the other.
The tone and vibes to these films are totally different but they remind me of each other in terms of the human experimentation aspect, as well as there being a whole special ops team of people trying to track down and kill a genetically modified human-thing. Young-nam Jang is also featured in both films.
The Childe and Seobok are similar in that both films center around protagonists with unique backgrounds—one a mixed-race martial artist and the other a genetically engineered human—who are pursued by powerful forces. The protagonists are also protected by skilled guardians. These protectors also a crucial role in guiding them in some things.
Very different story but I was remained of Seobok while watching Alienoid. Both movies are sci-fi, has non-human main character, special powers, action and in both are parent/child relationship but they're not actually related by blood (in Alienoid it's Father/daughter relationship, in Seobok it's not entirely family-like relationship but I got fatherly vibes from their interactions)
