The cancer that is "figher/assassin"
Complete list of "figher(aka bruiser)/assassins" according to riot games.

Extended list of fighter/assassins

of that list, you've got
who aren't considered top be top teir in most cases. (by no means am i saying they are bad, in fact all 4 are quite strong, they just have more obvious and more easily exploited weaknesses)
The concept of a "tank" in league of legends is very vague. It normally means a champion who has a higher ratio of CC to damage than a fighter, without really any more resilience in their kit than your average fighter. The resilience of most tanks is actually almost purely a result of their items not their kit. Slowly over time of the game the line between tanks and fighters has been getting blurred to the point that most can be considered either.
So the line between tanks and fighters is vague in the league, though as the game has developed it's been becoming more blurred, aside from a few specific champions.

I believe it was Phreak who said that the archetype of tank/assassin isn't something that fits in the game. Which i would tend to agree, what the roles involve are complete opposites, generally speaking. Tank - low damage, very resilient, generally high control over a fight, low mobility Assassin - high damage, very squishy, singles out a targets, highly mobile
So with fighters becoming more like tanks, that means the fighter/assassins are becoming more like tank/assassins...
tl;dr tank/assassins are bad for the game. the line between fighter and tank is getting blurred fighter/assassins are turning into tank/assassins
is a great example of riot making a champion win a kit which has potential, and the community turned him into a support despite being show cased as a bruiser.
is listed as a marksman/assassin. Your point? It's called primary and secondary, but really the archetypes they are given is just to give players an easier idea of what their play style is at a glance.
is a fighter/mage, though he plays nothing like a mage really.
low damage