Riot Games chose to completely rework these two champions, weakening Twitch's stealth aspects in order to make him a more normal carry. Evelynn was completely reworked, and she is so much different from her old self that she's basically a new character. Both of these champions are still "weird," however; despite the changes, neither character exemplifies any element of the current metagame.
I think that this is sort of dangerous. Riot has already introduced one very weird champion recently that has turned out to be viable (if not strong), Jayce. The stealth rework patch will also be succeeded by Zyra, who has very weird caster/pet mechanics. With all of these dramatic meta changes, League will become very unpredictable, especially at the middle levels of skill where I reside. I think that this is somewhat of a bad thing.
Don't get me wrong; I am interested in the stealth rework, I think Jayce is pretty neat, and I really want to play Zyra -- if not for her hot pixels, then certainly for her interesting pet mechanics. However, I really dislike it when Riot releases a lot of "weird" champions in a row, as it does strange things to the metagame.
The last time this happened, it was when Viktor, Ziggs, and Ahri were all released in a very short period of time. We kind of dodged a bullet, as Viktor is generally considered weak and Ziggs is underplayed (I still feel he's strong), but Ahri changed the meta wildly and had to be iterated on several times to be brought in line. I don't necessarily agree with all of the changes, but if we look at just the impact of one "weird" champion, the changes are fairly far-reaching. For another example, look at the Jax rework and how it changed many of the top lane matchups; many people are still not accustomed to him, and he has a very high win rate as a result (he might also be overpowered, but I will not attempt to make that value judgment here).
Now, we're looking at four effectively new champions (and that's completely discounting the Xin Zhao rework) in a very short time, and heavy playtesting on PBE with Twitch and Evelynn ensures that they will be factors in the new metagame. It's OK to rock the boat a bit with new "weird" champions and strange mechanics, but four in a month's time is a bit overkill.
Even if Xin Zhao stays as a relatively normal alternative to existing melee bruisers, Evelynn's impact on the game alone will be huge. I know that the current solo top matchups are a bit strange, as everybody is still getting used to what Jayce can accomplish. I'm not sure whether he will remain a player up there in the long term, but he has a lot of powerful tools, and a lot of them are very different from what an average top lane champion can threaten with. If we combine that with the addition of Evelynn to solo top and jungle, Twitch to bottom lane, and Zyra to mid (or jungle, who knows), there are a lot of new threats to learn and practice against.
If you multiply this by three (plus Jayce's matchups), this will be a huge pain both for players to learn and for devs to analyze. Weird mechanics put a lot of stress on players, as they have to learn to counter all of these new pieces. When one of those pieces is a jungler like Evelynn, it makes it much harder, since you have to combine dealing with her along with any other things you're also trying to learn. It took me forever to learn how to deal with support Alistar in bottom lane. If you're having trouble with that, adding two new elements you know nothing about (like Evelynn jungling and Twitch carrying) can be intensely frustrating.
I hate Draven, but getting Draven and Varus was nice because they weren't overly complex to learn to play against. Draven was especially nice because countering Draven is less about dealing with his absurd gameplay and more about punishing him for trying to do his absurd gameplay. Laning against Draven is stress-free compared to a Vayne, Graves, or Sivir lane because there is much less I have to think about. I also liked that while Varus has some uniqueness, Darius is really straightforward and Draven is extremely linear, making learning matchups much less troublesome for the last few patches.
Even though I'm probably going to play Evelynn (I like her and tried to make her work in spite of her flaws), I would definitely prefer to have fewer big changes like this in a row. It makes it much harder on me as a player to learn counters. Although all of us would like to say that we are experts and we can figure out the counters right away, it usually takes us many games playing against something to get good at dealing with it. With weird champions, it takes many more, and with so many in a row, the burden is much higher. Good luck!
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