Earn Your Fun: While the beginner campaigns act as solid tutorials to teach the basic mechanics of the game, learning how to effectively manage your troops, which troops are best for the map, how to handle different enemy troops, using your hero effectively, and smart SP management are crucial skills that can only truly be learned through experience. Thing is, the game can be absolutely brutal at times as it expects you to have a firm grasp on these concepts for many of the later missions, with Gerald's campaign being the only exception since much of it is a glorified tutorial and designed to ease players in. This can make the game extremely frustrating at times, but also incredibly rewarding as the player is constantly learning with hard fought victory to show for it.
Growing the Beard: Crusaders was already a great game, but only with Heroes adding to the story did the tale of the Encablossa War truly blossom into a complex tale of war, politics and intrigue, granting many side characters more depth and filling out parts of the story yet unexplained.
Narm: the English voicework of Crusaders is a mixed bag, with some voices like Lucretia/Morene being rather well acted, while others like Hugh and Urukubarr are just laughably bad. Hugh especially sticks out due to how poignant his character is to the narrative of Gerald's story yet sounds like a washed up drunkard hanging outside a 7-11 instead of the admirable military commander he actually is. Heroes thankfully got new and with a few exceptions much better English voice actors.
Nintendo Hard: Crusaders isn't too bad actually bar a few of Gerald/Lucretia's missions and Kendal/Regnier who are purposely designed to be difficult, but Heroes cranks up the overall difficulty noticeably to where passing the first two missions on any campaign above Ellen(the easiest in terms of difficulty) is considered a milestone in itself. Even Ellen's campaign is noticeably harder than Gerald's about midway through, despite being the game's "easy" mode.
Padding: Unlike The Crusaders, where every mission is a story mission, Heroes has two Level GrindingRandom Encounter missions every 1-2 story missions. They are optional, but consistently not taking them will make an already very hard game almost impossible. That said, half of Regnier's and Kendal's campaign in Crusaders consists of story-free open battle missions, too.
Sequel Displacement: Few people have heard of its predecessor, Kingdom Under Fire: A War of Heroes, released on the PC. It's even more unique/weirder than this game.
The ending to Lucretia's campaign is surprisingly poignant, even without dramatic slowmo or sad music. It's just so cruel to simply end her campaign at such a depressing moment, even if one didn't like Rithrin. It's extra-bad if one has already played Heroes and knows how things play out from then on. Lucretia goes off to join the Kaedes and everything you fought for in her campaign ultimately comes to naught.
Gerald having to leave General Hugh, his Parental Substitute behind to die, in order to save the king and then, getting scolded by Rupert his comrade and life long friend for leaving their father behind.
That One Level: the infamous cave from Ellen's campaign(her 4th mission) has caused many newbies(and even Crusader vets) to drop Heroes entirely due to how difficult it initially seems and how quickly you can be overwhelmed by the ever spawning enemies. Even if you do get to the end your entire army can get wiped out by the two Dark elf archer troops casting blizzard which can result in a Total Party Kill depending on how badly you've been worn down. Mind you, Ellen's campaign is supposed to be the easiest one.
The Scrappy: Walden both in-universe and out. He's a massive Jerk Ass with an inferiority complex to Gerald, gets off of being a dick to him by insulting his dead parents, and fittingly enough heads a Spearmen unit which are considered scrappies in themselves due to being one of the worst units in the game. Let's not forget to mention he's a required unit in one of the story missions too, which of course he complains about.
It's somewhat mitigated later on, when after General Hugh dies, Walden forgoes his normal Jerk Ass behavior and acts generally respectful towards Gerald in his time of grieving. Not to mention he later commits a Heroic Sacrifice to buy The King time the escape.
What Do You Mean, It's Not Symbolic?: The games draw some pretty obvious parallels to the historical crusades (and not the idealized variant either, but the unforgiving war of extermination between races and ideologies), and one might also see a few similarities to World War One.