I have been able to land in trees, but some trees are dense or large. The bounce the chopper away and if you persist in descending will crash 100% of the time.
I also found that if can land near by and exit the chopper, you can walk to the victim on foot. Once near, the trigger will fire and victim will board the chopper. However if you walk more than ~0.1nm from the chopper it will first warn you, then fail the mission due to abandoning the aircraft.
I am unable to complete SAR missions due to these kinds of issues.