Now that's the best question I was asked about this tierlist and I'm happy to answer it. As previously mentioned, I hired an EXPERT in AI who also has a little of semi-pro gaming background and understands terms like KDR, KDA, APM and the likes as well as MMR distribution. But no matter how much objective data we crammed into it (the first version only had signature heroes, now there are all sorts of stats) everyone seems to always go "but it's just your opinion". The idea was to remove my own opinion and be a 3rd party observer on things that everyone can agree on. Things that actually happened. Things that can be verified, regardless whether or not you like or dislike a person in there.
I just want to give one example of things I did to fight my own bias. Before the AI ranking came, I did a social experiment: I placed people that are higher than they should be lower. And I placed two certain individuals that were just mediocre and told them they're S tier. What I found was that both parties were HAPPY about it. Of course, why wouldn't the C- tier noob be shown that they're in S-tier or top of A-tier. And why would the humble, nice person who's way better than what I've just shown them be any mad about it?