It has less to do with an explicit disadvantage and rather the evolution of the metagame that has lead to where we are now. Consider a few facts first:
- Unlike other MOBAs, LoL does not have a turn rate (champions can change direction instantly and therefore ranged champions can kite more easily)
- Melee assassins generally have more escapes/steroids/displacements (grouped together as they can all be used to escape/avoid damage)
- Glass cannon has been suppressed by the meta for a while now (ranged champions have inherent ways to cope)
Ultimately, it boils down to the fact that ranged auto-based champions are simply easier to right-click steamroll with. Melee auto-based champions excel when the player has the good planning, calculation and mechanical skills associated with playing melee assassins.
For example, take Yi. During Alpha Strike, you do not take damage and cannot be targeted. Couple that with the fact that it refreshes upon kills/assists and that you can kill squishy champions in just a few seconds. It boils down to joining the fight after AoE's, crowd control and other hazards have mostly been used. It boils down to having the mechanical and intellectual skill to use Alpha Strike to avoid damage rather than deal damage.
Another factor is the realization that whether auto-based or caster, they are all melee ASSASSINS. Melee "ADC's" don't really exist, in the sense of a champion who deals persistent damage throughout the teamfight. The role of an assassin is primarily to eliminate important targets before they expire - meaning death, escaping or simply running out of ammo.
Take Nocturne, who I personally build as an assassin despite his capabilities as a bruiser. Nocturne can land on your carries from across the map relatively quickly. This means positioning and presence don't matter. He can be somewhere earning gold while the two teams skirmish before a fight and be in the right place at the right time. This also means the removal of a lot of the difficulty in playing a melee assassin. Realistically, Nocturne just needs to land on your back row and eliminate them as they're exposed and unlikely to stand a chance in an effective 1v1. If even one carry goes down, Nocturne has done his job by absorbing the focus as a less-essential teammate while eliminating a more-essential enemy. An extreme example is a combo I like to perform with my duo partner. As I cast Paranoia, he ults to me as Shen. This prolongs my survival enough to likely eliminate both carries as well as delivering him to the best position for an AoE taunt. This is the epitome and apex of the planning and calculation required as a good melee assassin player.
TL;DR Melee auto-based champions are just assassins. This means a different playstyle from the right-click mashing of ranged ADC's.
