2% magic damage per max hp on a 5000hp tank is about 60 damage with mitigation from mr and magic pen roughly taken into account.
Procs three times, for up to roughly 180 extra damage in 3 seconds, which is about 3.5% of this tank healthbar.
Liandry is not the tank killer you think it is.
Magic pen% + good ap and lot of AH > liandry