So yeah, apart from batshit crazy totally game-legal balance stuff that @Chris_Gwinn was talking about (which, trust me, was awful to try to GM around – it really exacerbated PF’s already-substantial problem where it’s very possible to build perfectly great RP characters who are noticeably less useful in combat than others, taking it to its logical extreme, where encounters that were challenging for the badass Champion Path Mythic Characters were super unfair, one-hit killers for the social-focused characters), Mythic Adventures also happened to release in what I consider a particularly low point for Paizo as a publisher.
Their editing got increasingly sloppy and haphazard (lots of really silly typos, broken/nonfunctional rules that needed to be errata’ed almost immediately, etc.), and their ongoing support for new rules systems was really diminished. They’d drop a sourcebook with a cool new category of rules, a couple of PF Companion 32-pagers and an Adventure Path campaign that featured the ruleset. . . and then never mention it again. So you’d get something potentially very cool like Mythic, which had all sorts of great, albeit poorly implemented, ideas, the AP would show some of the potential for it, and then rather than expanding on those ideas or correcting the missteps, they’d just let it die on the vine.
It was a really frustrating time to be a PF GM. In retrospect, their product pipeline had outgrown both their staffing capabilities and the financial realities of their diminishing sales in the face of 5E, so it makes a lot of sense, but I always wished they could have gone back and “fixed” this stuff (see also: Occult Adventures, the Advanced Class Guide classes, etc.).
The upshot of the videogame is that they get the chance to take the cool, core ideas, and substantially tweak them. Going with the game’s more origin-like Mythic Paths instead of very genericized Mythic Paths of the book goes a long way toward infusing them with flavor, and I’m already seeing really great balance changes from the Owlcat folks that would decrease that “usefulness” gap I was complaining about above. Of course, given that last time around, Owlcat’s character-balance-tweaks seemed almost completely divorced from their game-mechanic-balance-tweaks (changes to encounter design and enemy stats, especially) at launch, there’s no guarantee that the changes they make to Mythic stuff will be helpful if they go and muck around with other game systems simultaneously. Hopefully they test all of that better this time around to avoid the major problems Kingmaker had at launch.
Also, yes, having the computer go through the convoluted math of exactly how many times your Strength bonus gets multiplied when you Crit with a Keen Heavy Pick while Mythically Vital Strike Power Attacking with an Overhand Chop on your Two-Handed Weapon fighter, versus relying on your player to keep 9 spreadsheet tabs running on their Surface they bring to every game to help with the math is a huge boon, hah!