Flipping a capital w/emis in 60's is extremely difficult and you're looking
at around t10 before most emis are good enough to have reasonable chance of
success.
If you have allies or your own agents, they can counter by going head to head
vs enemy agents or going after their char also, forcing them to pull back.
Also, upping loy in capital to 90+ helps enormously and usually causes enemy
agent actions to fail (along w/hated relations).
In a recent game, our side had multiple assassination failures (at least 15
or more) vs enemy char in his own capital that had loy 95. All our agents were
80 or higher, even had some over 100 w/arties. We had about 90% failure rate
total.