I'm more used to people just putting car bombs on the vehicles, though.

You should be good with the e-brakes, and should never 100% depend on your primary brakes when driving, but in practice I'm sure a lot of people get into situations where they wouldn't know to switch to e-brake if the main brakes failed, or wouldn't have time. The biggest risk to the assassin is that car accidents in modern cars just aren't that fatal -- you can hit another car head-on at 60mph and, with seatbelts, non-offset crash, airbags, etc., either walk away or at least survive at a hospital. It also leaves enough forensic evidence, especially if the driver survives and reports "my brakes just didn't work!" that it wouldn't be surreptitious.

A bomb isn't likely to be taken as an accident, either, but is at least likely to be effective.