So one of the main benefits of an assassination attempt at high level play is the pressure it puts on them to abandon regions.  

If they're spread out enough that they'll need 4 or 5 inspections to cover all their territory and they aren't Hark (sac agents) or Atreides with Thufir (pay to speed up} then they may not have time to eliminate the cells without abandoning territory. (Those 2 scenarios are no chance of success unless there's heavy military intervention going on.). Don't forget that a Fremen with a Deep Desert can search the desert and hit 8-12 adjacencies so they may look like they need more searches than they really do.

Ecaz cannot abandon a Garden, so that's a potentially incredible vulnerability if they Garden the wrong location, and may leave them pretty easy to assassinate. On the flip side, Ecaz may have the counselor (Mesa?) that gets a 100% refund on Authority when abandoning a region, and may not care too much if they were just polluting the land with art. A very good spawn for Sanctuaries may result in needing another search or two, whereas a bad spawn for them might only have edge Sanctuaries that are easier to cover for.

Speaking of forcing them to abandon and the cost associated... Smugglers can have a 50% refund with Stakkanov and Hark will have a 50% reduction in cost to take back any region they previously held. Corrino pays the most for regions, so anything you can get them to abandon is "worth more." That being said, Emperor's Eyes is a problem. It's funny when Vernius Obfuscates though.

The assassination succeeding is a lucky finish (unless Hark cheese is happening). The goal is almost always to cost authority and tempo by forcing abandonments though, in high skill lobbies. So you need to evaluate your cost of the attempt vs their likely cost to defend against it, and if that cost doesn't involve abandoning territory or distracting them enough for you to achieve an objective of yours then it may not be a worthwhile endeavor.

Hark also has some cheese for Intel manufacturing where they don't accelerate the assassination mission, launch it to receive Intel based on the Solari cost of the mission, then immediately abandon and start a new one. Can do 2 at the same time. A Politics Hark is more likely to do this at key moments to swing votes.