The best part of this pattern is probably the construction order. If you've ever sewn anything that's fully lined, it can be a pain in the ass to turn right side out. The way Morgan does it is similar to the burrito method when sewing a shirt with a yoke.
It made it so I could do most of the construction before Mikey tried it on. But I could still easily make all those adjustments to the shoulder and side seams. So if I were sewing for someone far away, he could come in for a fitting near the end of construction. And even a slow sewer like me could get it finished that day.
Thread Theory has you interface the entire front of the vest to add structure. I think it also added to the ease of the welt pocket construction. I used cotton fusible interfacing.