This feedback is based on my "Heroes of Hibernia" playtest campaign.
So far, I've run multiple adventures covering 2nd-level/1st-tier, 4th-level/2nd-tier, and 8th-level/4th-tier over the course of a month, and mythic flaws have yet to be a factor at my table. (Two of my players have dependencies, one has vulnerability to cold iron, and another has hubris. There was actually a slight possibility that hubris might come up, but the PC with hubris wasn't the one exposed to any of the fear effects included in my playtest adventures.)
Granted, I could have rewritten encounters in my playtest to play up the mythic PCs' flaws every now and then, but that would have been contrary to my playtest methodology: I'm trying to see how the mythic rules play out when used with adventures written before the release of the mythic rules. Accordingly, nothing in any of my playtest adventures was changed after the fact to accommodate the mythic rules.
Under those circumstances, mythic flaws chosen by my players didn't come up even once. I should add that this didn't bother anyone at my table in the slightest. Given the circumstances of my playtest, I was running a game where the PCs effectively had no mythic flaws, and no one at the table felt we were missing anything.
(The interesting flavor associated with dependencies and hubris can exist just fine without any mechanics. The PC at my table who chose dependency upon alcoholic beverages was still a drunkard, the PC who chosen hubris was still a braggart, etc.)
I can see mythic flaws being included in the rules as an option for the groups that want them, but during my playtest, mythic flaws have proven largely extraneous to the mythic experience.
