These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Arc Fatigue: There is some of this problem in the Cool Ranch books. These were actually the first stories written when the writers thought that each world should have five books. The writers even realized that looking at the same landscape for that long would be boring so they helped mitigate the problem by adding another book that takes place elsewhere and giving the haunted skyway a major makeover to make it more of a ghost story.
Striped bumbaloons can vengeance strike anyone that hits them up to three times which makes it a chore to keep your Swashbuckler alive.
Frogerales are annoying at the Castillo Sapo dungeon, but at the Frogerale Warden, they can take down a level 50 swashbuckler with their constant spamming of overwatch. At the end of cool ranch, Bloody Bob Barnes appears on every ghost ship you fight and LOVES riposting your attacks. Especially problematic when you get cancelled out of an attack string with a companion like El Toro.
Mooshu gets even worse.
This time they have Samoorai Champions and Archers. Samoorai Champions get lots of free hits due to both first strike and vengeance strike. Samoorai Archers love pelting you with arrows and seeming to get a free bonus attack whether they hit or miss.
However, they don't even come close to Tengu Sorcerers. They can hit multiple targets with one attack, then spam the primary target with a string of mojo echo procs.
Marleybone has two particularly brutal ones.
Armada Dragoons. They love spamming The Big Guns half to death so you and a few companions are at about half health when you're playing with a 4-player group and you're focused down.
Radical Dogs. A minor one, but can beat you down by getting a relentless string. Especially problematic when they start beating down your musketeers.
Aquila has the most out of all of them.
Manticores. They can lock you in a relentless string just like radical dogs, but they are quite dodgy and can riposte attacks for bonus damage.
In the same vein as Armada Dragoons and Tengu Sorcerers are Saytrs and Ophidian Flame Dancers. They may cause some trouble when there's only one pirate, but a four player group is going to result in a bloodbath. This is especially true with Ophidian bosses like Mari and Chryssida.
On another note, there is the fight with Count BrastilloDe Brass. The boss himself is just annoying to say the least with him casting Rain of mortarshells on you and dominating musketeers with Rank 2 Quick Draw and in the second fight, Rank 2 True grit. (Basically Musketeer Vengeance Strike) However, It's all worth it in the end when every time you hit him, he makes the most Over-exaggerated sound that is both hilarious and satisfying.
The Scrappy: Miracle Mitch. In the recent update, now once your companions are defeated, They are put into bed rest for up to six hours at high levels. The only way to revive them early is through Miracle Mitch. The Scrappy part comes when he charges HUGE sums of gold, making dungeons much harder as you can't revive them immediately with mojo potions.
Tear Jerker: Bonnie Anne's third promotion, in which absolutely NOTHING gets resolved, and if anything, stuff gets WORSE. It tackles her family issues and immediately sends her into Woobie territory after what she had to do and what it cost her family. It doesn't help that it's really easy to get attached to her as a character due to being rather down to earth in terms of personality.
Privateers have Valor's Armor. Using this in combonation with valor's fortress can turn literally anything into a Tank.
Buccaneers have their infamous vicious charge. It lets them move all the way across the map to deliver high damage to a target and absolutely destroy their Accuracy for a few turns. After, they get an additional attack to an enemy nearby which does half damage, But can't be dodged.
Swashbucklers have Assassin's strike, which, with buffs, can nuke squishy targets. Oh, and with the right gear, it can be spammed multiple times.
The Woobie: In her final promotion, Bonnie Anne's brother gets angry at her for taking a job with Special Branch (which you completed for her previous promotion). This is the same Special Branch that her brother says is enslaving their friends and family. Bonnie tells him she took that job to fight the Armada, but he doesn't listen. The kicker though is that after Guy Fox escapes, Bonnie explains that he's probably going to hang for his actions. Talk about family issues.
Subodai can also be perceived as such during his final promotion, in which, while seeking to win the title of Khan, he becomes the unwitting pawn of Qulpa, Batu, and Guyuk's plan to get rid of Hunan, Great Khan of the Amber Horde, and effectively finds out that these Khans he's been looking up to his whole life are good-for-nothing tyrannous scoundrels, rendering his entire life a lie. I don't know about you, but that sounds like a pretty big downer to me. Oh, and did I forget to mention, his FATHER AND GRANDFATHER were also Khans. Yippee.