Not really. I've had at least 6 assassins so far in this campaign that reached 10 Subterfuge. What I've found is that I have to start on captains. You're right, it seems much harder to take out Diplomats and Spies than it was before the patch. The 10 assassin only has about a 50-60% chance of killing a spy with a 5 Subterfuge, and unless he has a few of those "increased chance to be assassinated" traits, taking out a faction leader/heir is really hard.