I lunge in a pessoa and I do find it helps my horse work across his back - it definitely helped to build his muscles up when I was bringing him back into work after 6 months off
Lunging is a lot more demanding on a horse than working in a large arena. I wouldnt use it on a young horse at all. I dont find it helps much with softness, flexion, lightness, or balance. Building up muscle and getting fit yes, but personally anyway my horse is out 24/7 so he can do those things on his own time. I much prefer groundwork and nlateral work to improve a horses way of going. Oftentimes a horse's shoulder can be jammed (on forehand, hard to stop, motorbikes around corners, trots like a pony) and while lunging wont do much for this, 10 minutes of lateral groudnwork can have a huge positive effect and get the horse a lot more free, flexabile and supple.
I use a pessoa on Jess and side reins on Flo. Jess has pessoa to encourage her to stretch and soften and to build up muscles in the right places, and she leans on the side reins. Flo has side reins to help her come into a contact!
I don't lunge my horse in anything. She works hard in her ridden sessions and is working well in an outline. When I linge her I like her to be able to have a stretch and move freely.