I upvoted, but I'd provide some clarity here.
Assassins are high reward because they have an insanely strong lane presence, and generally can 1v1 any lane opponent AND have tools to roam and get kills elsewhere if their lane opponent won't fight.
Assassins are "high risk" because if they don't get an early lead with that big advantage, they become much weaker late game. If an assassin doesn't get kills in the early-mid, they can't 1 shot the carries in the mid-late, meaning they continue to not get kills, and since they have no utility outside of killing someone, it generally puts their team at a disadvantage.
i.e. they're not high risk in the sense that their plays get punished by them dying and their lane opponent getting fed. They're high risk in the sense that if they personally don't get fed, they probably aren't going to win the game.
I upvoted, because this gameplay style just sucks for the person not playing the assassin though.
I can outplay you every time, but since you're innately stronger in the early-mid it doesn't matter, I won't develop a lead. Which means even if I deny you repeatedly, all it takes is my bot lane to give you a kill to get you rolling. Or my bot/top/jg to lose their lane and your ally to carry you because I'm not getting kill gold (because you have no 1v1 risk) to carry against them AND I can't roam on them because you'll kill me if I run into you on the way there.
If the enemy team has an assassin (or 2, or 3) your team's best strategy to win is to literally turtle from level 1 and give up farm and play super safe to ensure you don't die, and you'll outscale and win easily. However that gameplay pattern is horrible and no one (especially in solo queue) would ever actually do it!