Winrate is really the only legitimate method for a non-riot employee to determine a champion's strength, as we don't have access to any better data. And it's a pretty good one. People will invent reasons like "bad people play lee" to explain his terrible winrate, but guess what--bad people play every champion. Bad people play sejuani too, yet something about her makes it so they still win. Diamond and Master lee sins who play him in ranked STILL have a garbage winrate.
People can theorycraft and make up their own reasons for why someone is too strong, but it's just talk with no actual data behind it, and usually is chock full of cognitive biases (ever notice that's it's always the flashy champions that get complained about the most?). People get wrecked in a highly visible way by some champion a couple times and think it's broken while forgetting all the times the champion they are complaining about had little to no impact at all. It's also easier to ignore the strong, but more subtle power held by other champions (sej is a good example--not so flashy, but incredibly strong).
As for people suggesting that pick and ban rates be taken into account as well, they have literally nothing to do with champion balance. They are more measures of perceived champion strength rather than actual champion strength. What the average player thinks and what is true are two separate things. Maybe lots of people think lee is strong, and they pick him all the time, but at the end of the day he still loses substantially more often than he wins. It's just herd mentality ("other people play him so I will play him"), especially when the LCS is involved ("they did it in LCS which means it must be good in silver too"). Pick and ban also take into account other factors like annoyance (maybe fizz wasn't op, but he was annoying) that are completely unrelated to champion strength.
