Well first we just need to understand what an overloaded kit actually is. And a simple way we are able to do this, is asking ourselves what each ability inside of a kit really does and how it does this.
Take for example, Thresh compared to a champion such as Janna.
Thresh passive gives him infinite scaling for his Armor and Ability power, giving him two edges rather than one standard edge most champions not designed by Mr. T.
Thresh has a Q, this hook is capable of displacement, strong cc, playmaking, scales with his infinite AP, he can use his hook to save his life by transporting himself to non potential threats. This is only his Q.
Thresh also has a W, this W is capable of many things such as shielding a team member from damage including ignite and AOE damage. Team members can click this lantern to get out of harms way, secure a kill, position, fake bait, real bait, takes facechecking out of the question while most supports are forced to get into harms distance to place a ward and possibly get caught. This ability also scales infinitely, and at late game is very comparable if not even better than Lulu and her shield due to the Olaf treatment she had received.
Thresh has an E ability that is capable of displacement, slowing down the enemy to secure a kill or protect a teamate, this ability can be used offensively and defensively. This ability scales infinitely with AP and has high synergy with AA's, it allows Thresh to become a poke champion and can do effective damage that nullifies the enemy laner from farming effectively as they may normally had against a Janna or Lulu for example.
Thresh has an ultimate that has real damage and a 99% percent slow, to take this into perspective this slow is slower than Nasus slow and can also be applied multiple times. The damage scales with his Passive, and applying the ultimate is as easy as landing a hook without any form of risk of dieing before you are able to do so. This ultimate controls AOE forms of strategy on the battlefield that is very good peel as well as very good offensive play. This ultimate can hit multiple champions multiple times, and often times hit MOST of the walls.
Janna has a local (It was nerfed from universal) passive that gives 5% movement speed to allies around her in a very short distance.
Janna has a Q that can knock up enemy champions who are foolish enough to walk in front of the obvious trap, and has terrible damage, scaling and needs to be wound up 100% if it was to be considered an actual damage threat.
Janna has a W that can slow enemy champions who get into threat distance of the ally adc or Janna herself, this scales horrifically into AP and lasts for a brief period of time, very similar to a Lulu Q.
Janna has a E that can shield your ally teammates from taking any form of damage (I may be wrong, does True damage ignore shields?). This scales very well into the late game and is a force to be mentioned while team fighting, such as "Will i be able to actually kill this adc with Janna around? She is not a threat, but if i don't kill the adc i know i will NOT be able to escape." The exception being Fizz, Zed and other champions with overloaded kits.
Janna has an ultimate that pushes away the enemy team, healing your team for a good ratio and can be used tactically to take over a dragon or perhaps a baron after a team fight this the enemy team may have survived enough to contest if it did not heal the enemy team.
Now look at the two things these champions provide on the battlefield, who has more capabilities and who is more versatile as well as having more of an overloaded kit. Now, would it be jaw dropping for me to say that Janna is unhealthy for the game and Thresh is not? I do not agree with anymore Thresh nerfs or anything else, he is in a perfectly fine position and should remain where he is-however he WILL be picked always because he simply brings too much to the table for other supports to actually begin to keep up with. It is like trying to play against a professional in SC2 whilst you are also a professional, but the difference being a 50% handicap on your edge.
Riot thinks these champions are unhealthy for the game. Also, these champions will be nerfed even if they are literally dead in the ground.
Janna (Did you know that Janna is getting nerfed?)
Lulu (I miss her)
Sona (Did you know that Sona is getting nerfed?)