Feast or Famine actually isn't bad design for Assassins and here's why

It's a good thing that a champion will either be good or bad depending on how the player plays in a game. If a champion was equally useful no matter how fed or behind they were they would never be played unless they were way overtuned because it wouldn't be worth it. If they were undertuned then they would never be played because getting fed on them wouldn't do anything and if they were overtuned they would be played nonstop because no matter how far behind they were they would still be able to carry games with how op they are. If if they were in the middle ground they wouldn't be worth playing because there wouldn't really be any risk or reward in their gameplay at all. No risk of falling behind and no reward for being ahead.

Being feast or famine puts a lot of risk and reward into their gameplay unless they are overtuned or undertuned. It's risky playing an assassin because if you fall behind and get destroyed during laning phase then it's really hard to make a comeback. How does it feel when you completely roflstomp someone in lane but then losing because they were still able to be useful without having to try that hard. If you lose lane as an assassin you can still be useful but you just have to work harder than other champions to do so which makes sense. You shouldn't be rewarded for losing you should be punished. This isn't a kids game where little Johnny gets a trophy even though he's terrible at baseball.

Now for the other way around. If an Assassin does well during the laning phase and can snowball then they should be rewarded for how well they did. They shouldn't be at a static value if usefulness no matter how well they did because that would be very unrewarding and boring to play. Having the feast aspect in their gameplay makes them risky because they can fall behind but also makes them rewarding because they can get ahead and snowball a game to win it.

One of the good parts about assassins is that they can still be easily shutdown even when they are ahead with just a little bit of teamwork. A Khazix can get as fed as he wants to but it might be a little hard for him to do anything in teamfights when you have a Lulu ulting the ADC or Lissandra cc locking him down. Take a look at this game for example. Early game I got completely destroyed by Ziggs and friends camping me but I was still able to make a comeback because I played smart and got kills off of enemy mistakes. Then I was able to carry my team and we started to win fights. However, because the enemy team had champs like Rammus and Fidds the famine aspect of my gameplay was hard to pull off and well eventually lost the game because, despite me being fed, I could still be dealt with through cc, grouping, etc.

The feast or famine aspect of assassins may change based on their state in the game. If you look at Season 3 Akali pre 3.5 you can see that there was no famine aspect in her kit because no matter how far you put her behind she could just roflstomp your team anyway. Today, I think that she is pretty balanced and in a good middle ground of feast or famine because even when behind she can still catch up and even when ahead she can still be shutdown. In both cases she is heavily rewarded for playing well and heavily punished or playing poorly. This is how assassins should be made and not just "You stomped me but that doesn't matter" or "I destroyed you but that doesn't matter".

Now this is not to say that some assassins have problems with how their kits work and how fun or unfun they may be to play against but just that Feast or Famine actually isn't a bad thing.

Downvote away or whatever.