Statistically, AD assassins are doing much better than AP assassins.

Please hear me out before you downvote. This is primarily a statistical analysis, where I discuss rammifications at the end.

In terms of raw win rates (champion.gg), this is the order of assassins:

1. Talon: 53%
2. Shaco: 53%
3. Rengar: 51%
4. Zed: 51%
5. Fizz: 51%
6. Evelynn: 50%
7. Katarina: 48%
8. Kassadin: 47%
9. Akali: 47%
10. Ekko: 46%
11. Leblanc: 46%
12. Kha'zix: 46%

Average win rate for an AD Assassin: 51% Average win rate for an AP Assassin: 48%

But let's say you think general win rates are too specific, and you want to look at win rates from the most experienced players only.

1. Rengar: 59%
2. Zed: 57%
3. Fizz: 57%
4. Evelynn: 55%
5. Talon: 54%
6. Shaco: 54%
7. Leblanc: 54%
8. Katarina: 54%
9. Kha'zix: 52%
10. Akali: 52%
11. Kassadin: 50%
12. Ekko: 46%

Average win rate for AD assassins: 55% Average win rate for AP assassins: 52%

But let's say you don't like champion.gg statistics because you think they're too small a group size, and you want to look at win rates from Lolking's ranked section.

1. Talon: 52%
2. Rengar: 50%
3. Zed: 50%
4. Shaco: 50%
5. Fizz: 50%
6. Katarina: 48%
7. Evelynn: 48%
8. Ekko: 47%
9. Kassadin: 47%
10. Akali: 47%
11. Leblanc: 45%
12. Kha'zix: 45%

Average win ratio for AD assassins: 49% Average win ratio for AP assassins: 47%

Rundown AD assassins have higher win ratios across the board in every catagory. Of the five AD assassins, three are doing extremely well (Shaco, Rengar, Talon) one is doing average (Zed) and one is doing extremely poorly (Kha'Zix). Of the seven AP assassins, one is doing well (Fizz), one is doing average (Evelynn), four are doing poorly (Akali, Katarina, Kassadin, Leblanc) and one is doing extremely poorly (Ekko). On balance, AD assassins are doing fine in terms of win rate with the glaring exception of Khazix, while AP assassins are doing poorly in terms of win rate with the glaring exception of Fizz. Assassins also tend to have higherskill caps then regular champions, as manifested by substantially higher win rates by players who use them more often.

The conclusions I draw from this are: -AD assassins may need help, through means of a new item because their itemization is a little wonky, but on balance, AD assassins as a class don't need a class-wide buff; rather, Kha'zix is just doing poorly and needs a buff. Rengar is doing fine, Talon has always done fine, but due to him having a poor laning phase and a massive power spike afterwards, can feel a little unsatisfying. Shaco is doing much better due to his ability to secure Rift Herald, and over all less wards works well for him. Zed is about average, not overbearing, not underbearing, but as someone who doesn't play him, I don't know if he has feel issues or pacing issues. Kha'zix is just not doing well at all atm. -AP assassins do need help, maybe not through a new item, but in general. Fizz is doing fine, could actually be a bit overbearing right now and might need to be toned down. Eve feels like she's lacking in power, but statistically is doing well. Katarina, Akali, Ekko and Kass all have very feast or famine playstyles that tend to be a bit skewed in the risk to reward ratio, and also tend to rely more on opponents making mistakes than players having skill with them, resulting in them having lower win ratios. If anything, their patterns need to be changed to be less feast or famine, rather than raw buffs. Leblanc is statistically doing poorly, but has the perception of being strong by the community. Her skill cap is fine (in my opinion) but her skill floor seems to be too low. Ekko seems to be just too weak as an assassin, primarily because his burst pattern is situational and has so much utility that actual numbers are toned down.

Tl;DR AD assassins statistically do better than AP assassins. They don't need a new item because they aren't powerful enough, and the entire class shouldn't be catagorized by the one person who is doing poorly (Kha'Zix). They may need a new item because itemization for them is a bit unclear and wonky.

AP Assassins need help at the moment, because most of them have kits that are outdated and extremely feast or famine. Statistically, they are doing poorly, and a large portion of the AP assassin cast is largely unused.

The problems with the AP side might not be able to be solved by new itemization, but they are in need of work more than the AD side of things.

Thanks for reading.