I think that mixing sustained damage with burst damage isn't a good approach to champion design.

I've analyzed the different popular competitive picks or often complained champions and I realized that most of them offer flexibility in the way they inflict damage :

• Gangplank : He's probably the biggest offender. High-damaging barrels which ignore armor for heavy burst damage, and sustained damage in the form of auto-attack, passive and ultimate.

• Fiora : She offers a lot of flexibility. Her cooldowns are really low, especially on Q. Her passive offers nice true damage bursts, and her ultimate allow her to burst down someone with a huge part of their Hp as % true damage.

• Mordekaiser : His play pattern is now focused on getting the 3 Q procs, which offers him nice sustained and burst damage.

• Riven : Her very low cooldowns allow here to burst down squishies while still being a tank buster thanks to a lot fo armor penetration and AD steroids

• Vayne : Well. Silver bolts offer huge chunk of damage. Even though this isn't really burst damage, it's pretty close.

• Diana : Also one of the biggest offender. She can blows squishies with her combo, but thanks to her passive and build path, she's really effective against tanks.

• Garen : His burst is conditional, but still available. His sustained damage apply mostly to the villain.

• Darius : His bleed, low Q cooldown and free armor pen lets him do nice sustained damage. His ultimate is definitely the definition of burst.

• Azir : Same as Diana, being auto-attack based allows him to be extremly effective in both damage types.

I think that, with the juggernaut era, being able to damage both tanks and squishies is a key to being viable. Everybody always complain about mobility, but I believe that this mix of damage types is a real power creep that needs to be adressed.