Hi Jerrica
It's difficult to generalize, as the risk of damage from a rapid full clearing depends primarily on the skill of the electrologist, secondarily on the equipment and thirdly on hair density.
With accurate insertions the power used can be kept low. Provided that the machine settings can be adjusted accurately the damage can be contained within each individual follicle, the blood circulation in surrounding tissue remains unrestricted and healing can occur from all directions. The skin remains visually perfect.
If insertions are sloppy the needle may not positioned where it needs to be. This means that higher settings are required to kill each hair and results in more tissue damage. If hair growth is dense the damaged areas may overlap, making the healing process less three-dimensional, slower and more difficult, and scarring may result.
Even coarse hair can be safely cleared in one go if the density is low enough, and even thin hair may need to be thinned over several sessions if the individual hairs are in close proximity to each other (especially if the growth direction is not uniform.)
Again provided that the electrologist is skillful, the total cumulative time required to achieve final clearance should be the same whether the hairs are destroyed section by section or using the thinning approach.