Mages are healthy, Assassins aren't.
Wil try to keep it short.
Mages are encouraged to farm it out until 1-2 items, then at minute 12-14 when they are lvl 9-11 and have their level 2 ultimate [the moment where they power-spike hard] look for objectives or group with junglers/roam to get a kill or 2. - It's common to see a mage with 120+ farm at minute 16-18 nowadays.
Assassins are encouraged to "get kills", play the lane, hit a combo, ignite > get a kill, once ignite is back up > do it again or roam bot lane. Most assassins have trouble clearing waves or farming in general, and it's common to see assassins with less than 100 cs by minute 20 or even 30 in rare cases.
If the worst case scenario happens and the **mage **player dies once-twice to a miss-play/dive/gank - there's still the chance that the player will farm out the difference and come back into the game.
If the worst case scenario happens and the assassin hasn't "popped off" and isn't 3-0 / 5 - 0 by minute 16-17 , chances are you're never coming back into the game.
Please do not promote assassins mid lane.
I very much dislike how if there's a -

the game revolves around them to the point that depending on whether they win or lose the lane, you can see the outcome of the game. - Those champions that NEED to get early kills to scale are just terrible to play with or against. [probably excluding kha'zix since he's always fed [sg-miss-fortune] ]. It doesn't matter if you as a jungler/bot lane/ top lane do well, if your mid lane is an assassin or someone with 0 utility and damage only, as long as they're bad players in general or didn't get early kills, the game's lost instantly, unless the opponents are the same way.
No the changes to the AP items/mana aren't going to suddenly make all players, pick only assassins, the mana bars are already high as it is with manaflow band / tear / rod of ages, but the indirect/direct nerfs to
might drag out or make for more one sided games. - finishing games, before you've stacked the tear and have little to no power to fight with. The buffs to AP on other items to compensate for its %ap from total %mana nerf due to the mana bars shrinking and therefore its on-use effect
and
...And yeah, I can see how annoying they can be if your team does not have some anti-assassin mechanics in play.
and then fades when she gets gunned down by the ADC. - Probably ,because she has to auto-attack to deal damage, where as Katarina has such insane base damage on her passive that it's unrealistic to not "pop off"