I personally didn't like the MYMU- I mained Swain and Cassy mid and, when I played support, I would probably play Zyra 60% of the time and I don't like any of their updates. It seemed to me that the update was very sloppy, like as if the updated kits were pushed out without having thorough testing and understanding of how their kits worked together or syngerized together. On a couple of the updates (Cass and Zyra), I can see that Riot implemented the ideas/concepts that the League community had regarding the champs, but the implementation was poorly executed.
Swain
I mained Swain (mid) for 3.5 seasons. I understand Riot wanted to give more skill to his simple click/point kit, which had already been relatively well-rounded for several seasons, but the new update is almost unplayable. His Q is unreliable now, and he already had a nerf to his W snare time. Swain is designed to be a lockdown/tanky DOT mage, and he can no longer accomplish that well. His weaknesses, low mobility and low attack range, make it difficult to take advantage of his 20% damage amplification in E at all levels. His damage is supposed to be DOT, so rather than increasing the damage on his Q, Riot should have increased the tether strength, either through grounded effect, slow%, or length/range to syngerize better with the rest of his kit. Increasing his damage on his Q is nice, but it doesn't help to syngerize with his E amplification/combo if his targets can't be locked down, especially with his low mobility and low attack range. The adjustments with Q were supposed to help his farming, but any skilled Swain farmer should learn to farm well, and I'm not sure that gaining a little bit of farming assistance is worth losing the ability to lockdown and trade with opponents.
Cassiopeia
I mained Cassiopeia for 1.5 seasons, after I decided that I should try to learn someone else besides Swain. Riot implemented all the changes that the league community wanted (poison-focused, less Twin Fang) despite that they warned against it. I really believe that any player that dislikes the Twin Fang aspect of her kit really lacks understanding how her kit fits together- she is a high skill high reward champ, and has potential to deal a lot of sustained damage if played well (kiting). It really makes sense for her Twin Fang to follow up on her poisons, and not just poison spam everyone for her damage.
The rework makes me really sad, because it is definitely less cohesive than the previous iterations. She just doesn't play better than either of the older versions. What I expected from the rework was that Riot would keep the same mechanics (cooldowns/ranges/etc) and shift more of the numbers/damage toward Q/W and away from E, which they did, but they also changed W/E a lot and nerfed the range on Q (which they are reverting), which should have just been kept. I'm sure that most of the players that gave input to her rework direction probably won't even play the new kit that much, and those of us Cass mains who loved playing her can't stand the new kit.
Zyra
The changes on Zyra don't seem to be that big, but in reality it does affect her game play a lot. It was really nice that they gave her plant AI the 'integllience' it needed to focus on champs. However, while she really needed a new passive, I don't believe the new passive is incredibly better than the old one. I do think that she should have potential for growing/spawning more plants, but the new passive is not the correct implementation of it and it shows Riot's lack of thought and sloppiness, especially because it's just using Illaoi's passive. A passive should complement a champion's kit, but it shouldn't be an extension of one of their abilities (and if it does, it should just be a passive on that ability). Like some other players have suggested, and as I also think, her seed carrying capacity should increase with rank in W, and I also think that generating more plants around her should require more skill than just depending on an RNG passive. Additionally, I think it's a little awkward and illogical for her plants to have both a timed decay and a health bar- it should be implemented into one bar (like Azir's tower- a decaying health bar that you can accelerate their death by hitting it).