It feels a little bit like cheating for me to draw on my knowledge of Scion: Demigod and Scion: God when critiquing Scion: Hero's system chapters, but I can't pretend to an innocence I don't possess. Scion as a whole breaks in some pretty spectacular ways, and though it's not quite unusable yet, you can already see the cracks.
The biggest flaw in Scion: Hero is that Epic Dexterity is so much better than anything else you might want to buy, and by quite a large margin. Especially if you took the totally broken "Untouchable Opponent" perk (and it is to my lasting shame that I, in a sense of misguided loyalty, defended it on message boards for far longer than reasonable; with the clearer perspective of distance, it's shocking that such a thing made it through editing). It's such a good choice that it pretty much boils down to a binary choice - do you want to be a combatant or not. If the answer is yes, then maximum Epic Dexterity is a must.
But it's important to note that, as of Scion: Hero, Epic Dexterity was merely disruptively optimal, allowing a character to have a consistent baseline that was roughly in line with a lesser character's peak performance. It could be overcome. You wouldn't want to bet on an Epic Dex 2 over an Epic Dex 3, but it's possible that the less dexterous character could have other advantages that would tilt the fight in their favor. By the time you reach Scion: God, that will no longer be true.
However, this particular unbalanced choice is just the tip of the iceberg. Scion: Hero is full of trap builds, overpowered bombs that look harmless on the surface, and basic mechanics that practically force players into particular builds.
Take the combat system. Its foundation is taken from Exalted 2nd Edition. Despite that game's notoriety, its fundamental mechanics actually worked pretty well. It only really fell off the rails when powered individuals got involved (which, admittedly, was not a small problem). What's fascinating about Scion is that it only makes a few small tweaks, but these changes cause the system to fall apart in completely different ways than the issues that would plague its inspiration. For example, the change to soak (damage resistance). In Exalted, soak would subtract from raw damage, reducing the amount of damage dice you roll. In Scion, you roll your entire raw damage and then soak subtracts from the successes on the damage roll.
Maybe it's not entirely clear why this should be a problem . . . until you take into account that the average character's damage and soak is not significantly changed between the games. So without doing anything special, every PC, enemy, and monster in the game has double the damage resistance it would have in Exalted. You also have to consider that unlike Exalted's soak system, Scion's could reduce the effective damage of an attack to zero. In practice, this meant that it would take an optimized fist-fighter to consistently take down average noncombatants with unarmed attacks (and mirror-match brawler vs brawler would have a predicted fight time that approaches infinity as their absolute skill level increased).
I haven't even gotten into the character creation system yet, whose point-based approach allows PCs of wildly different power levels right from the start, and which is so unbalanced that the order in which you spend character creation points can net you significant amounts of experience points down the line. Or the fact that a serious mismatch in point costs makes Epic Attributes dramatically more cost effective than Boons (the thematically appropriate powers of the gods, like Zeus' thunder or Hermes' ability to fly). Or that non-physical attributes and abilities have no robust systems attached to them, requiring GMs to basically bullshit their way through, say, a 10 success Intelligence + Academics roll. Or the way that a straightforward reading of an otherwise unobtrusive Boon would seem to suggest that it could provide double-digit dice pool bonuses for a span of weeks.
Or . . .
Look, this game has a lot of problems. I would not blame you if, based on this post, you chose to give this game a pass. It's probably the right call, honestly. Because the one thing that a simple rundown of the mechanics can never capture is the electric feeling of what it was like to be there. You are telling new epics for a modern age! Look at the geometric progression of the Epic Attribute bonuses and imagine how awesome it's going to be when you're character is a god! Check out that sweet-ass cover with Eric Donner looking all buff and determined, surrounded by lightning and crows, holding an implausibly huge revolver while standing in front of a skyscraper!
What I'm saying is that it took me far longer than it should have to even notice these flaws. I wanted to believe.
Which, I suppose, is thematically appropriate.