Deathstroke and Shiva were in the game, yeah, but they were used terribly. Bossfights against Bad Guy Batman With Swords and the best fighter in DC should have been amazing, but instead they just pop out of nowhere, go out like scrubs and then are never seen again (except for Deathstroke doing pushups in his cell, I guess). I spent the entire game expecting Deathstroke to come back out for a real fight, or for the next chapter of the multi-part Shiva sidequest to unfold, but it never happened.
Cannot agree with this because of Deathstroke and Lady Shiva. But the fact that I don't remember too many of the other assassins is proof you're not far off the mark.
See also: fighting the world's greatest sniper in a tiny room where he saunters around calling you names the whole time.