You are spot on about an advanced caster being able to cast under a tree limb or with brush just behind your elbow. An excellent caster should be able to make a cast of 65 feet (possible longer if conditions were right) consistently with both floating and sinking tips. In fact, I feel that an intermediate caster should be able to make a Perry Poke of 45 to 55 feet in the same situation.
I know of a spot on an Olympic Pennisula river that requires you to make a cast of at least 70 feet (80 feet is optimum) to get a good fishable swing. The problem is that you have to stand up to your waist in the river, there is a drop-off right in front of you preventing wading any further, tree branches overhead about 11 feet above the river, behind is a high bank with small trees and shrubs, and slack water where the swing ends. To make the cast in this spot, you must make a side arm, double spey, with sort of a modified perry poke to send the cast the needed 70 to 80 feet across the river. Not many bother to fish this particular spot; but for those who can make a cast of sufficient length, this spot willingly gives up its bounty during summer and fall.
You are also correct about advanced casters and distance, not all casts are suitable for 100 foot casts.