This was originally going to be a post about something different, but then I found League of Graphs so no need for me to look at side advantage on my own like I was going to (blue side has a small advantage in professional play).
So in replacement, a few thoughts with unfortunately no current statistics to back them up:
It seems likely to me that instead of KDA what you probably really want to look at is kill differential, or some other similar stat (where you are adding numbers instead of multiplying). My intuition here comes largely from baseball, where K/BB is actually quite bad compared to K%-BB%, even if the latter isn’t exactly mathematically sound. In baseball actually often you go one step further and just use FIP or xFIP. My guess is a similar metric would be better for actually evaluating League of Legends players, since KDA mainly increases by not dying, just as K/BB mainly increases by not walking batters. Like not walking batters in baseball, not dying is a good thing, but it’s not as much of a good thing as KDA would claim.
Of course this isn’t actually used by Riot because KDA is better for giving to your playerbase; it will never be negative (by definition), and it’s usually a number noticeably larger than 1, so it probably makes more players feel good when they see it in their in-game profile.
Since we’re nearing playoff time, this is also a good time to bring up a post I wrote up on LiquidLegends last year about regressing observed winrates. Immortals is a very good team, but they don’t have a true-talent 94% winrate against LCS competition. Estimating with this method gives about an 80% true-talent winrate. So, they’re still definite favorites against anyone they play, but certainly not unbeatable.