Assassins are generally considered inefficient. Using them for early expansion is usually much slower than recruiting other units to just conquer the provinces, and later game most any investment in items or empowering is cheaply countered by sticking some bodyguards around your commanders. You don't want to invest too much in assassins (or really have an "assassin strategy"), but that said there are a couple situations where assassins are useful.
As a surprise. Several assassins taking out the critical commanders of an invading army before your opponent allocates bodyguards can be worth their weight in gold. Just don't expect that to work more than once.
Seducers can be worthwhile even if they frequently fail. Grabbing an enemy mage which expands your magic diversity is immeasurably valuable.
Mage assassins are often useful not only because they're obviously more effective at killing, but also because they're useful for stuff outside of it. Even most mage assassins are going to have trouble dealing with simple bodyguards so they remain most useful as a surprise, the difference is you can keep several around waiting for an opportunity without feeling wasteful.