Unfortunately, even tier lists that are fairly accurate are fundamentally flawed in any competitive game, and they're especially flawed in League. At their best, tier lists show characters who do well in many situations or who are very difficult to counter. At their worst, tier lists are flawed and show a lack of understanding on the part of the author.
This doesn't mean that there aren't strong or weak champions, but a tier list doesn't even tell part of the story. It just gives an arbitrary rating that says very little about a character's true strengths or how to leverage them.
No matter the game, tiers are wrong
I play a lot of competitive games, and a lot of them have tier lists. 1v1 games are especially prone to them, but any game with asymmetric play generally has a list of strong or weak options.
Even in a 1v1 situation such as a solo lane, tiers are very misleading. As an example, one character (we'll call him Character A) has many good lane matchups and is generally considered top tier. On the other hand, Character B has fewer good matchups and a number of bad ones, but she has some specific counter to Character A. Character A might be top tier and Character B might be low or mid tier, but a tier doesn't say that Character B is a counter-pick and has reasons to be played.
Most 1v1 games use a matchup chart as a basis for its tier list, and the matchup chart is actually a much more realistic way to look at the game. We can see in a chart all of Character B's bad matchups and know that she might be bad to pick blindly, but if Character A is very popular and played all the time (or selected early in draft), Character B might be a good choice. Also, if Character B's bad matchups aren't totally awful, it might be worth practicing her and learning how to deal with the bad matchups in order to shut out Character A.
Unfortunately, a matchup chart doesn't say why a character has an advantage in any given matchup, and it might assume a particular build or emphasizing a particular trait that is not normally used much. In League of Legends, that might mean picking non-standard items or an unusual selection of runes, masteries, or summoner spells.
In all of its game modes, League of Legends is a team game. Character A might have good lane matchups, but with the right jungler, Character B might be able to secure kills better, or perhaps Character B is a champion such as Vlad or Kennen who doesn't need mana. This could be really helpful if his jungler is very mana-reliant, as is Maokai. Maokai can take the blue buff for himself instead of feeding it to mid lane, which will help out the entire team.
Teamfights also emphasize composition more than individual prowess, and a weaker pick like Miss Fortune as AD carry might work out very well in an AoE-focused nuke team. Again, tier lists don't show these choices; they focus simply on characters who are strong.
This is actually why I don't like the Sona nerfs (plus Sona is one of my favorite characters). The issue with her wasn't that she was too strong; it was that Ezreal was too strong and Sona is his best partner. If Vayne turned out to be too strong, we would see far more Taric or Leona picks, as they are good partners for Vayne and Sona isn't. I avoid mentioning Blitzcrank, since he is almost always banned in tournament play, but he is also an amazing teammate for Vayne (and probably needs to be nerfed).
In League of Legends, the design space is huge. There are over 100 playable characters, and none of them are very similar. When I give criticism to tier list authors, it's really not because I think they are bad players. LoL is just so complex that no one -- not even Morello -- completely understands it. This is why champions get buffed or nerfed a little too much and why Jayce and Zyra were released in such an overpowered state. Even the design team can't predict all the interactions that might be changed as a result of a new champion release or how certain skills might be used when launched into the wild. Even beta testing can reveal only so much.
When you make a tier list, you're making a statement about the balance of the game and about who beats whom and what counters what. Even if you're Morello, I think that you're probably wrong. Most people who have made tier lists only play a few of the champions placed in the top tiers, and a lot of the overpowered elements of a particular champion can be dealt with using skilled play. For instance, Blitzcrank's rocket grab is basically a guaranteed kill if he lands it, but if you avoid putting yourself into situations in which you can be grabbed or juke it frequently, Blitz is pretty weak. This doesn't mean that Blitz isn't strong, but it does mean that counter-play can reduce his effectiveness a lot.
I was watching Scarra's stream the other day, and he was matched up with a Nasus player. Normally Scarra has to carry his team, since his Elo is so high that his team is usually lower skill to compensate, and Scarra said of Nasus, "Every champion has [his] place... well, except maybe Nasus. I think he's kind of weak." His partner Nasus performed amazingly well, though, and won his lane decisively. Scarra said afterward, "Maybe Nasus isn't so bad; I don't know." Scarra is one of the most strategic and analytical players in the world, and even he admits that he doesn't know all the strategies. If Scarra doesn't know for sure whether something is weak, you can probably bet that you don't.
There is a certain class of player that actually can make value judgments and determine matchups. Unfortunately, most of the players who make tier lists aren't in this group. These players are the kinds of people who play matchups from both sides, who look at champion abilities and think about all the possible ways to counter these abilities, and who dilligently test to see what works and what doesn't. These players aren't always the best (they usually aren't), but they're the kind of people who view a game deeply and don't just cry "OP." If you're not that kind of player, you probably shouldn't make value judgments about a character. Most of the time, you're wrong -- and even if you're right, you're usually not looking at the whole picture. It's better just to play the game and learn to deal with whatever is frustrating you (maybe read a guide?), rather than cry OP and put Darius in god tier.
We understand what it's like to climb the skill ladder in League of Legends. The Summoner's Guidebook teaches you the tools you need to get a competitive edge. Whether you're climbing the ranked ladder, playing Draft Dominion, or getting crushed by intermediate bots, every enemy has a weakness. And every Thursday, Patrick Mackey shows how you can improve improve on yours.