NOT being able to 100-0 someone at lvl 3, Not even 100-50 at lvl 3 is healthy
And assassin's can't do that ATM unless you let them.
Base damage on QWER rotations for just about everyone is 1000 @ level 18, where base HP is 2,000 @ level 18.
Over the journey from ~500 HP to ~2000 HP that damage also starts out at ~60 and then gets ~55 base damage per level afterwards.
With AP you tend to need 150, 300, and 450 AP at ult ranks 1,2,3 to double your QWER DPS as well - and that holds pretty true regardless of who you look at. Including LeBlanc.
AD's tend to need less raw AD as AD is ~33% more expensive per-point than AP, and AP is much easier to amass in high quantities.
As-such: the goal of a 100%-->0% champion is usually to go mid-lane where you'll generally be up a level on your opponents (+~55 base damage vs. ~85 less target HP). Even with that: you also generally need to pure pure-glass to OHKO an opponent due to the damage investment necissary to pull that off, and you want to target an opponent that completely neglected any sort of defense.
With most assassins their R is extremely vital to their OHKO potential as well, preventing many from landing a convincing killshot pre-6 without poking down their opponent to ~50% first.
A fed assassin with a significant item/level advantage can OHKO a tank, but a fed anything can cause you serious problems.
Otherwise assassins stay at 100%-->50% if you bother countering their damage with defense, and they can only generally OHKO an opponent that's made the conscious decision to avoid building any kind of defense at all to play a pure-squish champion. Going glass comes with it's draw-backs that you're, well, made of glass and need to play around that.