So my group finally got ready to play Wrath of Nature. We were using the pre-update mythic rules, I'm afraid. Here's how it went so far.
Mostly this seemed like a normal encounter, which shouldn't be surprising, I guess. They've reacted much more strongly in the past to non-mythic, CR appropriate monsters using unpleasant tactics.
After they got mythic, a couple of people were confused about what their new powers did, but they coped. They finished the next encounter within two rounds, burning down the treant in one.
They were all pretty happy with their new abilities and I'm not sure they could possibly lose any fight close to that CR. The champion and the guardian were so tough that damage was mostly ignored and the spellcasters (both of whom could cast any low enough spell on their list as a swift action) could just kill the hell out of things on a whim.
The chasm was a different story. They struggled like hell to beat it. Two out of four characters were in full plate and the others has no dex or acrobatics. When they tried to fly over, I started up the wind and had them make fly checks or get dragged downward. Nobody had the skills to get through and the party ended up at the bottom of the hole discussing plans. The lightning horrified them, so they tried to fool it with illusions. When that didn't work, they used the mythic invisibility cloak and I decided that must surely be enough to avoid it.
If they weren't mythic, they'd certainly have lost at this point.
Eventually, after much discussing and rulebook checking, the cleric spammed stoneshape to dig their way up to the hydra's den.
Overall, I'd say it worked pretty well.
The players had fun and were able to deal with very high powered encounters without seeming out of place.
They might even be too strong in a fight, but I'm not really sure. I'll reserve judgement until we finish the adventure.
I'm not convinced the party should have been able to beat the chasm at all, but I was pleased with their solution when they did.
Interestingly, the old amazing initiative ability, while incredibly powerful, didn't hog mythic power at all.
I'd say it made the tier 1 path abilities of the martial characters nearly pointless, but everything else was used about as much as I would expect.
When we switch to the newer rules for the final encounter, I can confidently predict that the players will stop spending on amazing initiative completely in favour of their tier 1 path abilities.