Everyone knows carnivals are scams, but this one seems exceedingly poor if all you want are the two new Grimoires. Outside of that, increased rates to Riders and Ninjas is always nice, but 2x isn’t a great bonus. Perhaps the greatest shame is that it won’t overlap with a godfest.
As for the quality of the new monsters, Ars Nova and Goetia are better than they look. The most important thing to know is that randomly spawned orbs do NOT replace orbs of the same color, meaning you’re guaranteed to get 3 extra orbs of that color. Combine this with an orb enhance on a potential 6 turn cooldown and you have a fairly powerful skill. Their leader skills are underwhelming, but they actually aren’t bad for non-god REM rolls. Their awakenings aren’t sexy, but the offensive potential of orb enhance awakenings is always underestimated particularly when combined with their active. Everything combined makes for a solid non-god REM roll, but I’m pretty sure most would still rather have a Ninja of the appropriate color.