Fair my fucking butt. It's all RNG in the end. Someone could have grinded during the entire event and spent 200 AP potions and not gotten a single rainbow chest to drop. Meanwhile, someone who only went through 50 or so runs might have gotten extremely lucky and gotten 4 summons to drop. There is no guarantee that you will even get one to drop so the "if you grind long enough you don't have to buy all four" isn't true at all.
Asides this, in all honesty, I don't think the drop rates for this event are fucked but the AP cost is way too high for what few mats you're getting. It should have been 10/15/20 rather than 10/20/30 for Beginner/Standard/Expert for the amount of materials you get. 20 would be great since it would allow you to do more runs, increasing your chances of getting a rainbow chest to drop and, even if you don't get a rainbow chest to drop ever, would at least allow you to exchange enough mats by the end of the event for one maxed out weapon or eidolon, assuming you no-lifed during the entire event.
I ran the Expert event for about 12hrs, or about 11 times. I got 6 hearts. I've gotten 22 hearts total during the event, having run it nearly non-stop for 2-3 days (I ran the Light Expert Daily for most of Friday).
I'm averaging a little under 1 heart per run, getting no hearts about a tenth of the time and getting 2 hearts a little less than that. Only 1 rainbow chest, with the SR bow. Very steady rate of enhance items, I've gone from 12-13k starting the event to 17.5 now, and up to the last couple runs I'd been able to level one or more weapons and one or more eidolon each run. Epigraph drops have been highly variable and mostly scarce, so I'm gaining SP slowly and doubt I'll have the epigraphs left over to get anything other than consumables after that.
Crap. I was about to go to bed. I guess I'll try it.
4 Golds (3 hearts and 1 SR chaos spirit), 7 Silvers, 8 Bronze. Hard to pass up.
RIP sleep.
They MIGHT be doing it on and off all day, like with last Sunday and the other "extra" events. I'm gonna make sure to at least grab the 25 or so I need right now just to be sure though, haha.