Overlapping springboards and mines are too frequent. Maybe consider making a new object out of that like a springmine that combines the effects of both.
Max shooters at 94% effectiveness, max melee attackers at 94% effectiveness... what am I forgetting? Oh yeah, a seemingly immortal elderly couple assigned to search (no one else, those youngsters just get in the way!) and their grandson assigned to the assistant role. After that it was essentially coasting to the end.
Nice to be rid of checkpoints (finally). The Monster City back wheelie challenge needs a re-balance though. 10 seconds eats away a significant amount of time in a level that's already deliberately short with it. I can pull off the 10 seconds necessary but find that I'm always in need of a spare 3-5 seconds necessary to actually make it across the damned finish line so the challenge registers completion. Even if I cut every corner, grab every accelerator and go as high as I can with the boost it still ends in a failed state since I can't cross the finish. Either a time extension on the level overall or a lessening of the time needed for the wheelie would be massively appreciated for that.
Difficulty curve goes from medium -> impossible which is immediately going to turn away anyone not familiar with these types of games. Having a bigger group of slower songs to begin with and then transitioning over to the faster, more complex key combo songs would have been a smoother balance. The same can be said of nearly every Super Crazy Guitar Maniac installment though. You get one song you can possibly get close to perfect on (if you're willing to practice on it), then the rest end up putting your fingers through the wringer.
Could use a stalking cue for when the ghosts may be directly behind you or in extremely close proximity. Some way to elevate the actual fear of running into them and spooking a player enough to think before they just spot one and immediately start backtracking.
Railgun shot is easily earned once you've unlocked the shotgun. After you have, play on Annihilator mode and wait for the crowds of zombies to start pooling together. Once you see a crowd big enough, take a shot and hope you clear enough of them out to get it.