Most the assassins in the game are not required to plan out what they are going to do and wait for the right time to act on those plans. They can initiate, assassinate, and escape on a whim.
exc.
It's a very complex issues due to years of development and game design, but part of the problem is flash being such constant power house among summoner spells, and mobility creep in general. If you are an assassin you have to be given enough mobility to actually keep up with your target, though some of that bleeds over to escape utility when you are given to much.
One option to removing such a break in the balance of the meta game is to increase the amounts of long duration CC, though that doesn't feel good for the champions who are getting destroyed because they also now have to deal with the new increased levels of CC. The way to fix that issue would be to add more options for tenacity, that have little impact on your damage, that way assassins are less effective for building it, but also must pick their targets and timing more wisely to not get hit with large amounts of CC when attempting to do so.
Currently assassins feel more like a blood thirsty, killing spree, fantasy, because you can get away with killing just about anyone without getting caught or punished for it. A real "assassin" would be along the lines of
where you are fairly easy to kill once spotted, but you are intended to wait on the outside of a fight until the enemy is distracted or dispersed, then find your target and kill them (probably a carry). This is something i think riot's design team forgot a while ago, this is evident in
's ult. It blurs the line between bruiser and assassin which is not a healthy spot for the game. You don't want champions who can both be in the front lines of a fight and assassinate a target.
I believe it was Phreak who said that the archetype of tank/assassin isn't something that fits in the game. The irony is that statement is that there very few if any real "tanks" in the game. Most of the tanks are simply bruisers who are called tanks, because of a slightly higher ratio of CC to damage.
So if a bruiser is effectively a tank and a tank/assassin shouldn't exist in the game, but the line between bruisers and assassins is getting blurred, then the game is moving in the exact direction which even Phreak thinks there shouldn't be any of.
search "assassin fighter" in the champion list when replying to this post, there are currently 19 champions (according to riot) which fit that archetype. Of them the overwhelming majority are considered over powered and/or impossible to balance with their current design.