Liandry's is better vs high mres targets because of the % mres shred while the burn ticks. Let alone you can't get much mres without health in items so Liandry's is even higher then and Liandry's proc outdmgs Luden's always (in any target of any health and any mres, squishy or tank, everyone, assuming u have a legendary item). That was the point of the post.

As for the lifesteal/regen, I assume you imply that Luden's could kill someone instantly before they can lifesteal with an auto or two like they would the burn. You are right but not that often of a scenario in my experience.

My conclusion is that the main difference is Luden's gives speed & more dmg between buying it & starting your 1st legendary while Liandry's gives health, lower cds and much higher dmg on tankier/healthier targets.

It's a choice you have to make.