It depends. If you're talking about basic mixers, the sort that get used for summing, these MUST BE in the build.
As for using external mixers at synthesizer signal levels for your outputs, this can be a problem, particularly if you're aiming to send synth-level signals into an A-D converter that's expecting to see standard pro-level (ie: 1.4Vrms @ 0VU) signals. These are almost certain to overload. There are analog mixers that CAN handle these sorts of levels, but the rationale for using present-day performance mixers in a modular setup is that you can automate many different aspects of your mix via modulation signals. Before these got more common, I would say that mixing via a small outboard mixer with the preamp gains rolled WAAAY back would be the right move...but when things like the Stereomix2, etc now exist, they're 100% the right way to go.