The difficulty of a mixed route can change from one hour to the next. A key icicle can fall off or a sudden warm spell can melt out much of the ice, making the route more difficult, or easier.
Mixed ratings are mere guidelines.
Realistically judging difficulty and safety is your responsibility. Correlating standard YDS free-climbing grades to the M scale is misleading. The best way to discern mixed difficulty is to get out and climb. Rest assured M7 will feel harder than M5.