I believe factors such as scarring, keloids, and hyper/hypo pigmentation depend largely on your personal genetic/hereditary factors:
[ul][li]skin type
[/li][li]skin pigmentation
[/li][li]family history[/ul]
[/li]Scarring also depends on cleanliness/after-care/hygiene after the electrolysis is completed.
The best area for a sample treatment is the target area. Everything else ist meaningless, because each area needs its own epilation parameters.
Regarding sensitivity: sense of pain and the ability to tolerate pain are different issues: related but not necessarily congruent.
Example: my pain tolerance is very low in the central part of my lower jaw toward the throat. Although my sensitivity is larger below my nose i usually can tolerate pain longer and better than in the firstmentionend region.