The most obvious limitation is RAM size. All samples for flex machines for all your songs needs to be in the sound pool at the same time (unless you want to change the soundpool on the fly which isn’t really practical). Add to this the maximum recorder buffer requirements of your songs and you may hit the RAM limit.
Another limitation are global settings, because all songs share the same global settings and reconfiguring on the fly isn’t everyone’s cup of coffee.
But beside these two limitations it should be as stable as with single song projects.