Ok, Here it my bitch with assassins.
1) There are 7 of them
2) 6 of the 7 are tier 1-3
3) Only need 6 to get to max stat bonus
4) You get to level 4 very quickly so you can start seeing 6 of the 7 very early in a match
5) You can get 3 of them by level 3 and that 150% crit bonus by level 3
6) Only Gunslinger has max bonus potential this early in a match and the max bonus increase isnt even comparable. For whatever reason Gangplank seems to be a fairly elusive Tier 3 from my experience. Assassins will shred through Gunslingers before their bonus makes a difference.
7) Only counter I can find is 6 Yordles. If the assassins have more 2/3 stars it doesnt matter. Even with 60% miss if the misses dont happen early and if the assassins are ripping off those 2k+ crits again it doesnt matter. I suppose a player can get lucky with drops and help counter it as well.
8) To go along with point 7, because of the low tier threshold once you get at least 3 early you can blow all your gold on rerolls and stack a team quickly and early in the match.
IMHO either move one of the assassins to another group and force players to wait out the Tier 4 Akali and move Zed or Pyke to Tier 3 to avoid seeing that 150% bonus so early in a match, gain win streak gold and keep your health bar up to give you more chances for rerolls. You can steal alot of early wins even when the 3 assassins are still 1 star.