Actually it’s easy. Any structurally damaged facility is dangerous to either dismantle or repair because further collapse could happen at any time; there’s no way for the engineers to really understand the load paths of the damaged structure.
The point is that gravity is working hard every second of every day on an un-designed structure in an un-designed = unpredictable way.
That’s why multi-story buildings are “imploded”. It’s a way to eliminate the vast majority of that gravitational potential energy (“PE”) from the structure and turn it into a simple rubble pile. And do so quickly from a distance.
As applied to Arecibo:
First mount charges where the surviving receiver assembly support cables attach to the 3 support towers. Blow them simultaneously to part the cables and the very heavy receiver assembly crashes through the flimsy reflector bowl and is safely on the ground.
Next mount charges around the periphery of the reflector dish. Blow them and the reflector falls into the pit too. Bring on the chopper-upper-crawlers to dice the wreckage into truckable chunks. Or, if the bowl topography is too deep / steep to work in, just leave / bury the mess in the handy pre-made landfill.
Seems pretty safe; none of the work need be done where / while there’s still high PE stuff overhead or uphill from the workers.