A serious problem with Lewis's account of counterfactuals is that as Elga and Pruss have shown, Lewis's similarity account licenses errant counterfactuals of the form "Were B to occur at t1, A would have occurred at t0", where t0 < t1. A quick argument for this conclusion is as follows. We have good reason to think the universe's future is longer than the past. On the grounds of divine revelation this is clear: the future is forever but the world was created a finite amount of time ago. On the grounds of science this is also likely: the universe is expanding, and either it will extend forever, or it will eventually reverse and collapse, but even in the latter case we're not yet at the half-way point. Therefore, it seems, a world that exactly matches our world in all of the future but in very little of the past will always be more like our world than a world that matches our world in all of the past but in very little of the future. Hence, if we look at worlds close to ours in which the antecedent of a subjunctive holds, we will do better to look at worlds whose future matches the future of our world, and then retrodict to the past from the antecedent of the subjunctive.
But here is an interesting partial solution to this problem. Suppose we abandon Lewis's counterpart theory. Suppose, further, that we assume that all of the causal history of an event is essential to the event's identity. (It is plausible that some of the causal history is essential, but the possibility of drawing a line as to what is and what is not essential is implausible.) Now if exact match of spatiotemporal regions of worlds requires the numerical identity of the events in these regions, it is impossible for two worlds in which causation goes orthodoxly from past to future to match in the future but not in the past. The asymmetry in causation together with the essentiality of origins of events thus induces an asymmetry in counterfactuals, ruling out most backtracking counterfactuals. Of course Lewis wouldn't have liked this, since it means that causation is prior to counterfactuals.
Of course, other problems for Lewis remain.