Energy has its own problems. It becomes weaker as the game progresses away from laning and come mid-late game, mana users have much more ease with resource managements than Energy users.
mana users have problems
energy users actualy dont, their kits are either made around burst gated by energy pool with quite cheap poke skills to fight after burst(look zed) or sustained damage dealers who just replenish their energy by commitement (lee siin)
none of those say that they run out of their resource in any other case than bursting someone down and you usualy cant capitalize on your ally or you being dead on top of assasins kit being made to be able to get away after that
energy unlike being completly manaless allows you to starwe yourself out if you play your cards wrong, its the only resource that perfectly gates your capabilities to your champs intended plastyle
the problem we face is not energy not having problems, its energy champions being able to put out the same amount of power in many diferent areas without trully paying for it since their resource regenerates and they dont have insane cooldowns
main pro of having mana is that your skills are more potent that those of energy/manaless counterparts because you are gated on the amount of times you can cast it before returning to base
when energy was close to beign free there was no need to keep the energy champions below their bar in skill potency since all of them were practicaly manaless
now mana was brought up to the place it was supposed to be in and all of the other flaws that came with mana being infinite became visible
the problem is mainly existant on midlane, other lanes dont hate it soo much