DDJ-RZ - Internal vs External mixing with RB

Is there any disadvantage using an RZ in internal mixing mode?  I like the idea of a separate headphone channel for preview which only seems possible in internal mode. Do I lose any effects or other features? For example, are all the effects like sound color and OSC replicated in the software?

Can I still quickly switch to an external source like an iPad in case of a software crash or controller lock-up? With the SZ I can just flip the input switch on a channel regardless of the state of the laptop software.

Can I have USB-A in internal mode and USB-B in external mode?

Mike