It's actually piss poor game design when every hero is clearly treated as if they are on an equal/comparable level because of the way the game treats hero progression. Everyone is homogenized into classes/roles, and even more homogenized because their recruitment is completely random (and thus implying any hero you get is at the very least, serviceable) and their level up progressions are entirely identical.

That kind of homogenization among heroes should have strengths in their design to accompany their weaknesses. But, like many F2P games, contain backwards game design in order to facilitate spending money to obtain what's considered "the best" in the game.

"Completing the tome" and "Maxing skill levels" is an example of poor game design via content padding. It's essentially inflating content by creating arbitrary time consuming facets of a game in order to increase the longevity of a game. It preys on the "achievement hunting" and "best in slot seeking" nature of people who play games regularly. The 1 gem bonus for maxing a hero out in the tome is completely arbitrary, it just invites players to spend mind numbingly obscene amounts of hours to get a single gem that you save and save for the chance to get one of the "better" heroes.

Pokemon has a similar function to "completing the tome" which is completing the PokeDex, but, again, there is no ulterior reward besides satisfaction on the player's part.

It's hardly comparable to Pokemon where there is no clear cut better/worse. It merely depended on the way you looked at the game. Weaker Pokemon like Dunsparce and Farfetch'd are sought after still because they're rare, therefore they're collectible, and endearing to many fans. These rarer Pokemon often have low stats and level up at a slower rate simply because they are not meant to be battle-ready Pokemon.

Whereas in Crusaders Quest, EVERY hero is implied to be a battle-ready hero of more or less equivalent skill with weaknesses balanced by strengths. Their flavor text implying each hero's ridiculous skill and mastery of their role says as much, anyway.

That said, no hero is inherently unusable. That's just petty. Some heroes are less effective than others, yes, but with the proper synergy and setup, you can use any hero effectively. For example, I use Giparang in a 3-block chaser team and he's consistently wrecking coliseum teams and hard mode stages.

Everyone is just jaded as to what's "usable" by defining "usable" as what's the easiest to use with the highest outputs possible.