I ran my group through the playtest adventure with some PC's we have from an ongoing game. Some background, first; There were five characters in the party, all at level eight with four mythic tiers. I scaled up the encounters appropriately, and for the most part, it went swimmingly.
We did run into a few problems/questions, though, and here are the main ones as we saw them.
First up, the Hierophant ability Sacred Boons. Specifically, combined with a cleric with the Animal domain, and thus, an animal companion. My player asserted that this would cause his animal companion to be affected, going up by (in our case) eight effective Druid levels, causing the stupid horse to gain something on the order of 7 HD and all that comes with it, two ability score increases, multiattack, devotion, and a few other things I'm forgetting. Is this intended? Because we all found it to be way, waaaaay more powerful than any other increase you could get out of a domain power. So much so, actually, that I forbid him the use of it.
Second, we ran into some issues with Amazing Initiative, but the already-posted changes to that seem to clear them right up, so well done there.
Third...well, this isn't really a problem so much as a hope. My players were disappointed with the range of mythic feats available. Almost everyone had a loadout of base feats that were incompatible with most of the mythic upgrades, and so felt left out. We made up some variants they could use, but basically, we would really like the list to be expanded.
Fourth, we had a question about a paladin's immunity to fear and how it relates to the hubris flaw, but with those relegated to optional status, I think that problem goes away too.
All in all, we had a good time with these. It seems to us a great way to make a game more legendary-feeling without spiraling it quite as out of control as the old epic rules. Good work, and thanks.
Hmmm... I see your point on Sacred Boon.
Cleric 8 (effective Druid 5)/Hei 4 adds 8 to his effective Cleric level for the Domain power, making him effectively a 13th level Druid. That's 6HD, +5 BAB, +3 to Fort and Ref, +2 to Will, some skills, +6 Natural Armour, +3 to Str and Dex, and 3 more tricks. Just the HD alone is double what a level 12 cleric would see.
I'd house-rule that secondary class effects (things which include "effective class levels") add tier only. I doubt that will break the bank.
I...um...well, I'm not sure what it has to do with this thread, but I'm also not sure which spell you're talking about.
If you mean the spell a spellslinger can sacrifice for the Mage Bullets class feature, I'd say it probably has to be one he prepared that day, but I guess it's a GM call.