The branch predictor should mean that in most cases the latency of branches should be constant. And if its wrong that's just as much a problem for an OoO machine as it is for the Mill.

For memory accesses yes, that's a big problem which is why there's so much effort in the design devoted to making software memory speculation feasible. The Itanium put some effort into that too but generally at a much higher overhead and the Mill's solution for dealing with memory faults in speculated accesses was frankly brilliant.