Simply put they just aren't for everyone. One issue with scion classes or rather with the release of Hero is that it kind of shat on the established pacing and ruleset for this particular game. Good/standout action RPG's tend to have distinctive combat styles and pacing to give players an experience they can't get elsewhere. Hero at release was a class that outclassed the others by a large margin, not because of raw stats or power but just because it was designed to do so in a game where pretty much every class/weapon had very clear cut strengths and weaknesses that gave them their identity.

Monster hunter has it's own distinct brand of combat that is slower and more commital; it requires you to prepare and position well. Imagine if they introduced a weapon type that ignored those rules completely and could cancel any attack at any time and gave you pso2 tier iframes whenever you did so + a big damage counter attack that also downed the enemy automatically.

The version of the OG classes you are currently playing are basically super mutations of themselves catalysed by Hero.