Starting with the Juggernaut rework, which is widely and well-deservedly considered a complete failure. The only 'successful' part of that rework is Darius, and not successful in that it correctly solved his problems or made him more healthy to face - no, just that he got a massive net buff and a huge boost to power creep. It's pretty much impossible for a competent Darius to lose lane now unless he is either camped or versus a competent Fiora or ranged champion, and was busted as fuck after a few small buffs, dominating the pro scene alongside GP and Fiora. He's since been toned down but is still utter hell to play against as pretty much every melee champion not named Fiora or Trundle and, while not as feast or famine, still very binary and problematic.
Garen was an objective failure and continues to be weak and underused in high tiers of play. Skarner's success shifts back and forth based on how strong the Cinderhulk-Trinity Force rush is for him, and his spires minigame is ridiculous and unnecessary. He's either pretty oppressive in the jungle or a shadow pick that serves as an ult bot. The items the update brought were welcome additions, but Steraks was pretty quickly abused by ADCs and thusly nerfed; DMP is pretty much a must buy on most top lane tanks; and Titanic has been repeatedly nerfed and is only seen on fringe users like TK and Volibear, who scale better with health than raw AD.
I could write a whole post on how god awful the Mordekaiser rework was, but you can ask the legion of Morde mains here about that if you want. It created more problems than it attempted to fix and continues to have massive repercussions today.
The ADC rework is my personal most hated. It took 6 ADCs that undeniably had problems, jacked up their damage and utility, and made them all ridiculously strong balance nightmares that continue to be problems over a season later. ADCs were perhaps too weak early in S5, but that is because Vayne, Kalista, and Jinx were so insanely dominant that nothing else really got played.
Quinn is a roaming lethality-crit-fuck-you-melee-champion monstrosity that dominated top, mid, _and_ jungle and is still incredibly unfun or fair to face, and abuses pretty much any OP AD build that comes along. Graves is a complete embarassment to the balance team, and while I acknowledge he had a boring kit before, his new one is a disturbing testament to both damage and power creep. He obliterates anything that comes close, abuses lifesteal like no one else, and can even nuke you from a screen away. Caitlyn is......Caitlyn is complete bullshit. Corki became a monstrous hybrid carry and continued on to be the best mid laner in the game even after repeated nerfs. Kog'maw's rework was so terrible that it had to be reverted, the only live rework to ever have done so. MF's rework wasn't all that bad on paper but succeeded in giving her so much free damage that she sees actual, professional success as a _support_. And, her ult one-shots teams. The mini-touchups on the other ADCs were nonfactors for everyone but Trist, Twitch, and Vayne: the latter two becoming even more oppressive than before. Rift Herald is a gimped objective that requires way more investment than what you get out of it.
The new items introduced and the tweaks to existing ones only served to massively increase ADC damage and skew favor towards them in pretty much any circumstance. Crit, armor pen, attack speed - it all went up. ADCs even got an item that allows them to get _30% fucking CDR_, when they are the class that arguably should have the worst access to it (since their auto-attacks are so strong). The minion and tower changes were dumb as fuck from the start and set in motion the snowbally, stat-check meta we have today - the fact that a surrender@15 option even has to be considered is just sad, really sad. I fucking loved taking Clairvoyance (on Bard) and Clarity (on Vel'Koz back when he was mana hungry) in certain cases and I still miss them. RIP to Flask, Elixir of Ruin, Zephyr, Poacher's and Ranger's, and of course Stealth Ward and Mana Potion. You guys will always be missed.
The MYMU did a few small good things, but it also fucked up balance royally in the vein of its predecessors as well. The elemental dragons were cool for a second before you realized how much in free stats they gave and how stupid Ocean and FIre were (Earth helped fuck towers even more too). Instead of helping combat snowball, they only exacerbate it. The new items aside from the overtuned Morellos, the injustice done to Grail, and the additional nerf to Zhonya's did nothing to help mages and the hextech items are too niche and gold-inefficient to truly abuse. The shift for mages from mana to mana regen was a huge slap in the face.
Vlad, Malz, Swain, and Syndra were busted as hell for months after the update. Malz and Vlad in particular continue to be balance nightmares Riot has no clue what to do with. The problems Veigar and Annie had weren't addressed (hint, they still blow you up with little to no counterplay and then die, but Annie at least got 40% damage reduction), Fiddles is garbage now after one patch in the spotlight, Xerath hasn't really changed at all and Ziggs is Ziggs. Zyra is still a complete pain wherever she is. Vel'Koz was balanced before being touched but got his damage shot through the roof and is pretty sleeper OP now. Cassiopeia was arguably the only decent update.
Plants are just dumb and give too much hidden power. They are completely unnecessary and I'm still wondering which Riot employee came up with them in a coffee-induced stupor. Lethality is a complete and utter mistake and has broke the game for patches now. Talon, aside from his disgusting level 2, is arguably the only successful rework. Ekko had his most glaring problem fixed but is still a mistake of a champion. Rengar and Kha'Zix are disgusting and so was LeBlanc for patch after patch before she got gutted. Akali was unjustly nerfed but her design is so ridiculously unhealthy that she needs more changes. Zed is still Zed, except now he has Lethality to abuse. Fizz and Shaco seemed weak at first but now they are pretty strong, even more so for the latter. Fizz still jumps on you from a whole lane away and bursts the hell out of you, and Shaco still pops up out of nowhere to shank your ass, except now there are no pink wards (hello massive indirect buff to Vayne who was already overperforming!). Twitch was abitrarily buffed for some reason as well. Katarina went from a pubstomper that was bad at high elos to a hypermobile monstrosity that crushes at all.
The slow and unexplained removal of vision options and increased availability of vision denial is troubling, as is the obsessive and disturbing fixation by Riot to see how far they can break a champion before nerfing them to the ground, or leaving certain power picks untouched while gimping others intentionally.
This post wasn't a big rant to bitch about reworks (well, yeah, it sort of was). It's a cautionary tale to Riot. None of the class reworks have been objective successes. They have all broken the champions they set about to fix even further and heavily gimped others. Plants, lethality, RNG dragons, tower and minion changes, ADC itemization, removal of vision options, removal of sustain for anyone that isn't AD, and edge of night were all huge balance mistakes. 60% of updated champions are even more problematic now than they were before and 40% either are gimped or weren't even changed at all.
Do not fuck up the tank rework. I hate Trump ~~memes~~ as much as the next guy but it is seriously time to Make League Great Again. Do not give Sejuani three AoE hard CCs like you did Galio, don't turn Maokai into a tanky Zyra, don't make Zac's E global and his Q a displacement like we know you want to. Gut penetration options on items and move them into champion kits. Buff defensive itemization, especially armor. Tone down oppressive power picks that have run the meta for months now. Do not introduce unneccessary additions to the game that only create more problems than they are supposed to solve, and do not stonewall our protests against them if you do. Listen to your community and help rebuild this game from the pit of power creep it has fallen into.