Does anyone else feel that lifesteal is a very frustrating mechanic?

I won't go into a full paper about every detail, but it's a very frustrating situation when you calculate how much damage you need to kill someone, execute it perfectly, but then are slapped in the face because the enemy auto'ed a minion and gained 100+ health.

This is especially frustrating (imo) on Jhin. Because of his kit, he can heal upwards of 500+ on an aa and makes an assassin main like me feel completely useless because my job is meaningless late game since my target can't die from an assassin.

Edit: I realize items counter it (in the example I based this off of our team had both a morellos and a mortal reminder). I realize that people think trying to calculate damage that close is either impossible or risky. My point was that a lot of preparation to kill someone can result in a huge failure because they happened to auto a minion and regain a lot of health instantly. Once the game hit's a certain point, no matter how fed you were as an assassin, more often than not a life steal build can negate your purpose.