Different gauges will stretch somewhat differently for the same load, which will make the load distribution in the wheel "interesting". But while not ideal, this is rarely critical - a bit like mixing batteries in something electrical. Many people have done it in a pinch - and indeed survived to tell the tale.

As for spoke breakage: Breaking NDS spokes on a MTB hub is perfectly according to prediction.(low tension, cyclic load, fatigue break) while breaking DS spokes on a road hub is more awkward. Those should remain nicely under tension and have a good chance of being long-lived.