YMMV / Under Defeat

• Awesome Music: This being a Shoot 'Em Up, of course you'll find some examples of tracks to kick butt too. Take your pick between the original soundtrack by Shinji Hosoe, the arrange soundtracks which take the original tracks and give them new sections, or the New Order soundtrack by Yousuke Yasui.
• Broken Base:
  • Some players are turned off by the game giving out multipliers for having less lives than the starting number at the end of a particular stage, explaining that this encourages suiciding twice at the beginning of stage 1-1. However, this is often countered by pointing out that the bonuses obtained from doing so are a very small fraction of the player's score and only matter in a world record attempt.
  • The HD ports. While many praise the New Order mode, a major point of contention is that the Arcade mode is not arcade perfect: The Final Boss has a lot of slowdown and there are differences in enemy patterns; while not particularly significant to those who have not played the non-HD versions, those who have may find muscle memory screwing them over. Those who dislike the HD ports prefer the Dreamcast port instead, as it is arcade-perfect due to the arcade version running on the similar-in-spec Sega NAOMI hardware.
• Memetic Mutation: Cone trees.^Explanation From a joke review of the game complaining about the lack of detail on the game's graphics. And yes, that's the "Go play Mars Matrix" guy.
• Spiritual Adaptation: This is pretty much the closest you'll get to Tiger Heli or Twin Cobra with 3D graphics. Like these two games, Under Defeat is a helicopter-based shmup with a military aesthetic and subdued bullet counts.
• Tear Jerker: Stage 1-5 and 2-5, the final stages of their respective loops, feature a war-torn city with the stage starting off with no sound effects and an initially-somber track (whether you're using the Shinji Hosoe soundtrack or the Yousuke Yasui one), almost as if the game is highlighting the consequences of war. Even in the overseas releases, which water down plot details, it's still a very sobering experience to see 1-5 for the first time.
• That One Boss: Stage 1-5/2-5's area 4 midboss releases pods that explode into bullet showers. There is a pattern to the pods: When a green pod explodes, you're safe if you keep your copter still, while red pods will require a slight dodge upon bursting. Once you learn this, this boss wouldn't be so bad...if the boss wasn't also firing at you with its guns at the same time.