Hit isolated targets or even completely whiff way too frequently (Anivia and Leona will occasionally ult units who are in the process of walking forward and miss entirely, Brand will target a unit with RFC resulting in no bounces, etc.). Tier 4 and 5 units need to regularly perform at a high level to justify their cost and rarity - if they can't, then why are they high tier?
If caster units can suffer from unreasonable levels of inconsistency, then auto attacking units should too. Maybe they should have a 20% chance of dropping their guns, blades, and bows for 5 seconds. Imagine how stupid that would be? Or maybe implementing this change would fit right in with the theme of massive RNG. Casters only deal damage via abilities and often only get 1 to 2 casts per fight, so whiffing an ult likely takes them out of a fight entirely.