One thing I can observe with mythic rules, is that at higher tier the glass cannon characters only get worse.
In my group, we ran a spontaneous playtest when I was going through some old files and found an old 3.5 epic level boss and asked if anyone was interested in challenging it. Now, this battle pushed the limits of the playtest to the limits, since my boss was roughly a CR 34 encounter by itself, so the one player with a character ready quickly added at +3 template on. Being that we were both martials (the character was Fighter 20/Champion 10, duel pathed Guardian), I expected this would be an interesting fight of see both the offense and endurance of a mythic compared to a 3.5 level 30 character.
The results were not what we expected. A quick look at the character revealed that even surging, the fighter had to roll a 18 or higher to make a save against any of my boss's special attacks. I was also coming in with 3 times his HP, and my AC was such that he had to roll a 7 to hit me with his primary attacks. My first attack was a blast attack, basically a sonic fireball, which ate 1/3 of his HP right there. He spent his turn charging me, where he landed a few hits using the power to move and full attack. One full attack by my boss later and the mythic was down. Absorb blow worked against one attack, but as a creature with several natural attacks it couldn't keep up with the sheer number of attacks I had, especially when combined with poison and bleed (And this was allowing the Guardian's DR to work against my attacks, since I didn't have 'mythic' levels despite my CR I wasn't a mythic monster). As one person who watched the playtest put it. "Forget Mythic, just give me 10 levels in a normal class"
The biggest downfall of high level mythic that we are seeing in my group is lack of save progression. The mythic saves ability is nice, but it ends up being worthless when you can't make the save in the first place. Surging barely helps, since if you roll poorly on the d20 even if you roll well on the d12 it isn't enough, and you've basically just wasted your swift action for your turn.
Needless to say, from our playtests we're seeing mythic character being far more fragile than normal characters at all levels of play.