There are advantages to the bomb. Take the point of view of the assassin. You could walk up to the car while it is unattended, wearing a disguise to hide your face. You’d superglue your fingertips to avoid leaving prints (without it looking obvious like wearing gloves) If the car is parked in a public area (parking lot, garage, whatever), there is nothing illegal or noteworthy about walking around such a place.
If no one is looking your way, duck down and plant the bomb. You don’t want to set off a car alarm, and you don’t have time to use a lift and shop tools to really hide the bomb. So it almost has to be in a place that is visible from underneath, at least if someone were using a mirror to check.
On the other hand, a cleverly made bomb could be encased and covered with grease, such that only an expert would know that it was not part of the car. Wiring it to the ignition would take too long - I think a cell phone remote detonator is a better idea, and it’s not very expensive for the would be bomber to buy a couple disposable phones.
On CSI, they always find the suspect’s bomb lab and match tool marks or some shit. A smart assassin would destroy any tools used to make the bomb after the fact. (and actually destroy them with a blowtorch or something, not just dump them somewhere to be found)
Anyways, it sounds like a bombing could be done in a way that the assassin gets away with it. No direct evidence would be traced back to him. Of course, in real life, what would happen is that he would have committed the crime under orders from someone else (say a crime boss) and a half dozen people would know he did it. Inevitably, the cops would bust one of those folks and they would rat out the assassin.
Compare this to using a gun. Suppressed gunshots are still pretty noisy. The assassin is right there when the crime occurs (instead of long gone hours ago), and so there are tons of opportunities for witnesses to see him or her. Now the assassin has to actually find the target, which means he has to hang around for hours waiting for him or her. When the target shows, it could easily be at a time when there are crowds of people (and potential witnesses) everywhere. For someone important enough to be “worth” assassinating, that person is probably important enough that he or she is never alone.
As for making sure the victim is dead, well, obviously that’s just a matter of explosive power.