Suddenly, all that discard, land destruction, and card drawing they've used has backfired, as they find themselves at a low life due to Necropotence, with two (or less) lands in play, no creatures, and no cards in hand
The ability goes on the stack before blockers are declared, which means your opponents have a window to respond
The most broken thing you can do with this card is to remove all the counters from your suspended spells, getting them quickly onto the battlefield or making your vanishing cards last longer
Hangarback is at its best in grindy slugfests, preferably on the ground where it can get a chance to block and unleash its children
This incarnation of the character, The Eternal Wanderer , is fantastic for Commander
This can be used to delay opponents, prevent permanents from being destroyed, or even blink them for additional ETB effects