Digimon Frontier

The "worlds" in both Anime share similar elements to them despite having different concepts. There is also the element of getting to come home and becoming a better person by fighting in these worlds to survive and the characters grow in maturity over time. 

Both later iteration kids anime from big franchises, both more spiritual than their previous generations with astrology and animal spirits.  

Both classic monster battling kids anime that see humans fight alongside their series respective intelligent creatures (Digimon & Dinosaurs). Both have similar aged protagonists, similar style humour and similar premises & themes. 

Both reinvented the wheel for their respective franchises. Both were generally well received by the fan-bases despite straying away from the established norms of what makes a Digimon or Yugioh series. Both could be considered the third generation of their respective franchises (if you count Digimon Adventure 01 + 02 as one generation). 

Both later generations and iterations of classic kids anime franchises. Both feature lots of fun and relatable characters in large fantastical worlds. Both casts of heroes are similar ages as both shows have the same demographic as their target audiences. 

Both series of larger kids anime franchises that are extremely popular (Digimon & Yugioh). Both focus on futuristic technology and have secondary virtual/digital worlds (the digital world & Link Vrains). 

A simple story in which human children become great fighters in an other world to save it from destruction by evil forces. 

The concept of transforming to get stronger and using the power of your friends to get even stronger is prevalent throughout both. The heroes in each anime are on journeys to save their (worlds) from whatever evil threatens them and adorn explosive battles that get even more explosive as the series goes on.  

Both kids anime that follow young human heroes who ally themselves with their series' respective sentient creatures. In both the kids join the cause/battle of their creature companions and face a common enemy. Both shows also have similar senses of humour. 

Both classic "Monster Taming"/"Monster Battling" kids anime. Both have great world building involving a wide variety of intelligent monsters who befriend the human heroes of their stories. Both notably have the human kids actually fight themselves in these series (unlike Pokemon or even classic Digimon were the monster pals do all the fighting).  

Both iconic children's anime from large monster battling franchises. Both used as marketing for toys. Both's main cast of characters are all young kids who participate in intense battles. Both have a wide variety of monsters with cool designs. Both have great OST's and character designs. 

Both anime have characters that transform into stronger beings 

Both show's have kids as the main characters. Both have a digital based theme (monster?hero's). They also fight other monsters or hero's and after they win they capture them. (Digivice/DISK)  

When I first watchied Mujin Wakusei Survive, I definitely felt an essence similar to Digimon. Both feature kids stranded in different realms than their own, surviving on their own means, while they bond and have different relationships, whether they be positive or negativa. Now, I am comparing it with Digimon Frontier rather than Digimon Adventure because I think the similarities can be seen more on the former. I think the ages of the cast in this installment of Digimon are closer to the ones on Survive. Digimon Frontier may essentially be a kids anime, but don't underestimate it, it can get very emotional. Granted, Mujin Wakusei Survive is aimed at a more mature audience, but I believe people who enjoyed one of these two can enjoy the other. read more 

their places took the same place, in Shibuya in ChäoS;HEAd we can see 1 boy with so much girls, but in Digimon Frontier we can see 1 girl with so much boys the enemy themselve also claim that they had surpassed "God", but it was totally failure that because they underestimate the heros/heroines the main boy fell in love with the main girl all chara combine their power to defeat enemy *nb: the way hero & heroines in ChäoS;HEAd combine their powers, it really resemble "monshou" (crests) in Digimon Adventure 

Both extremely well known kids monster battling anime. Both belong to larger franchises with multiple series. Both explore the concept of not only the monsters battling (once a standard in the genre) but also the human kid characters/heroes partaking in the battles as well themselves. 

Both have a very emotional side,you think that ni the begining, the anime won't turn out very well but as you progress you'll see how each character developes and you will find more hurtful truths 

Both classic kids monster battling anime. Both have similar aged protagonists/heroes. Both have cool varied monster & character designs. Both have great English OST's. 

Both iconic kids anime. Both have humanoid monsters. Both monster collecting/taming/battling series. Both have intelligent and communicative creatures. 

Both kids shows with ancient beings/themes. Both also have a variety of unique monsters with both cute and formidable appearances. Both also involve evolution/growth of these monsters.