TBH they're not really assassins (save for Deathleaper, who is fairly capable against support characters) but more like commandos. Build a list with units that are happy to go second when setting up after the enemy army has committed to its deployment (T-fexes, Exocrines), take a bunch of Lictors, deploy them first in hiding, and your opponent has got to deploy very conservatively - and the minute a critical unit is exposed, you can throw a bunch of re-rolled charges at it to deny it shooting next turn.
Instead of taking up a single force org slot, a trio of Lictors enable a HQ choice to give you an extra CP. Being solitary types, that might be handy once the Codex drops - they could be taken with a Magus to counter Aggressors and the like, without compromising a detachment's <Hive Fleet> purity.
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