Broken Base: Until their inclusion, the lack of leaderboards in the Windows port. Some felt that it ruined the appeal of the games, especially Cardinal Sins as it's primarily a score attack game, others pointed out that the leaderboards would quickly be obsoleted by cheaters/hackers. Now that there are leaderboards for the port and there are cheaters, the "there will probably be cheaters" camp has sadly been proven right.
Mirror Shield REH has a lot of health, has some very vicious patterns, and to make matters worse, when it seems like you've killed it, it goes into a second form that shoots out three-bullet spreads and gets faster the more you wear it down. Things go From Bad to Worse in Area 26, when you have to confront Mirror Shield RAH and ROH, who have the same attacks and are fought at the same time!
The boss of the aptly-named Wrath/Saturn stage in Cardinal Sins; it's a ghost of Magnificence with ever-increasing rank that can only be tamed by shooting at a mirror hovering around it, which also cancels bullets. The objective, as stated in-game: "Don't die." You're ranked based on how many times you die, with the S rank only being obtainable by not dying at all. Its most vicious attack is its green homing laser attack, which is nearly impossible to avoid if the rank is high up. Many an overall rank has been dropped below S-, the required rank to face all four forms of the Mirror of Cardinal Sins, solely because the player died too many times on Wrath.