So, perfection is not possible in life.
Given a small enough sample size, something can appear perfect. If the client has a 0.0001% chance of messing up at any given time, then you'd need almost 700,000 players to have a 50% chance of seeing one error.
So, why does the League client appear to "mess up" more frequently than other clients? Well, if you actually look at posts on the official boards of various games, reddit, etc. and take into account playerbases, it really doesn't. Other games may appear to mess up less - but they also are played less. Messing up 50 times out of 1 million players is the same "failure rate" as messing up once out of 20,000 players. However, those 50 players will claim "omg look at this another 49 players had a fuck up but this other game only had 1 total!!!!!!! WOW CLIENT SUCKS"
League has been the most consistently highest played game (i.e. not factoring in "fad players") for many years now. It only makes sense it will appear to "fuck up" more often than other games, even when the client is just as good as in other games.
Now, I'm not saying it may not be possible that the client doesn't have fixable problems. In fact, I'm sure it does - and you can see problems they're fixing in the server status page. However, having problems does not mean it's bad.