If a Rails application is using class-level methods and setting class-level variables of a singleton class, and multiple requests end up sharing that same reused process, there would be data sharing issues since those requests would be sharing the same object.
I've tested this out and it seems that Phusion Passenger does indeed reuse processes and data-sharing in this particular scenario is a concern, but I can't find any info on this in Phusion Passenger's documentation.