A lot of League's class balance problems stem from how overloaded mages are.
Mages do too many things too well as a class. Best waveclear by far. Burst that competes with if not exceeds assassins which are supposed to be the burst/one shot class except they can do it from range. Highest range in the game. Access to a great deal of psuedo-defensive itemization (and what was Riot smoking when they gave mages access to
)that also gives a decent amount of damage. Strong at all points of the game, and some are actual hypercarries
who actually encroach on the ADC DPS niche, except with more utility + range + burst added on.
So the knockon effects on other classes are:
Bruisers/fighters/juggernauts don't just have to compete with consistent DPS from the ADC, but psuedo DPS from the mage because unless they itemize heavy MR, they will be whittled down by skillshots and kited by mage utility. Assassins need strong MR options to do their jobs because nearly all mages have hard CC, and most have it outside their ults. Furthermore, because mages can not only oneshot a single target, but can potentaily oneshot an entire team, assassins become more and more redundant. Essentially, why pick a melee assassin if you can have a ranged one? ADCs HAVE to have "bursty sustained damage" because they have such a small window to work with when the mage's abilities are down, especially given their utility, defensive options and itemization. Also this isn't an option usually so they have to hit the frontline, which makes strong frontline killing tools a neccessity, but it's virtually impossible to be strong against tanks but not against squishies. Basically mage damage forces ADCs to have insane mid late game damage because mage damage forces them into frustrating and unsatisfying play patterns (focusing the tank). Tanks have to have attacker agnostic itemization (aka they have to have items that give both HP AND MR) or mages will chunk them out. It's interesting btw, that armor tank items tend to be gold inefficient, while MR tank items have had to be outrageously efficient (certainly old BV/SV were almost 200% gold efficient), despite there being more sources of physical damage than magic damage.
So something needs to go on mages. Either the utility needs to go, they need higher cooldowns, they need to operate at lower ranges, they need worse defensive itemization, they need less mana regeneration, or they need to do less damage (or have their damage gated more effectively, for example, Syndra should not 100% a squishy with just R) or some combination of the above.