Keep in mind I'm rating all these cards for Standard playability
You need some early interaction that can brick-wall an entire assault, and Hangarback Walker has the capability of doing that
Rhystic Study is one of the most annoying cards in Magic, but only if youre the fool not playing it
If a creature card has multiple instances of scavenge, you can activate either ability (but not both, as the card will be exiled when you activate one of them)
And you don't need to spend a lot to pick one up and try it out
It doesn't copy whether that permanent is tapped or untapped, whether that permanent has any counters on it or any Auras and/or Equipment attached to it, and it doesn't copy any non-copy effects that have changed that permanent's power, toughness, types, color, abilities, or so on