Tiamat's splash diameter, 185, is fairly small, reaching out a mere 92.5 units from the center. For comparison, most melee champions have an attack range of 125. If facing a Tiamat-heavy build on an enemy champion, staying even slightly spread out will counter it completely. On the flip side, abilities that force stacking like 加里奥的Idol of DurandIdol of Durand or 奥莉安娜的Command: ShockwaveCommand: Shockwave will make it much more effective.
The splash damage will also apply a percentage of total physical damage dealt, hence Sheen proc will also be accounted for the splash.