Regen
The Regen system has a significant drawback: its energy regeneration effect is extremely poor and rarely activates. Basically, for a Regen champion to regain energy, the unit must be at full health first. However, given the context of having too many champions dealing AoE damage dominating the meta, this condition is rarely met. Even at full health, the Regen effect only restores a minimal amount of energy, so players are forced to combine it with the Shojin Spear to improve this.
Despite having such a weak effect, the Regen system has not been reworked by Riot since the beginning of Season 5. The reason is that this system possesses an extremely powerful unit named Heimerdinger. If they find a way to buff the mana regeneration of the Regen system, compositions revolving around Heimerdinger would become incredibly unbalanced as this unit would continuously cast spells and deal excessive damage. Perhaps leaving the Regen system with such a weak effect is Riot’s way of balancing Heimerdinger.
Mage
In previous seasons of Teamfight Tactics, the design of effects for magical damage systems such as Sorcerers and Mages applied by Riot involved increasing a large amount of skill damage. This allowed units of that system to deliver burst damage at powerful moments and create a distinctive playstyle. However, Riot changed this in Season 5, causing the Mage system to provide very little skill damage stats.
Even after removing the limit on additional skill damage in patch 11.17, the effects of the Mage system did not get any stronger. Perhaps if it weren’t for Vel’Koz, this system would have been reworked a long time ago. The reason is quite simple: Vel’Koz’s basic skill damage is already very high. If Riot changes the Mage system to provide more skill damage, Vel’Koz would become too overpowered and very hard to balance.
Legion
Even after reworking the effects and receiving a buff in the previous patch, the Legion system still hasn’t found a way to break into the current meta. Compositions revolving around 4 or 6 Legion champions have nearly “gone extinct” for a long time and show no signs of returning. With so many ways to help champions gain attack speed and sustain in the current meta, the value of the Legion effect is extremely low.
The champion responsible for carrying the Legion system at this time is Kayle, with her ability to deal comprehensive damage. If you haven’t noticed, even in her first and second ascension states, Kayle already has the ability to deal extremely high true damage. This is a crucial factor that allows this champion to take down the heavily armored champions that are very popular in the current meta. Perhaps without Kayle, the Legion system would definitely be rated as the worst system in the game.