I know nothing about this manufacturing, but I watched in YouTube. It seem frequent design change is not possible with this type of manufacturing. Some of you may familiar with type of manufacturing. What is your experience?
Why would this approach be any more or less able to deal with changes than other molding methods?
I’m not terribly familiar with it but it would seem that it’s nothing more than a large mold. Any changes would/could be made in the same way you would other types of molds…except bigger 
I have worked in the roto molding manufacturing industry for 6 years. There are a number of differences in it compared to other plastic molding industries such as injection molding, blow molding and etc. A lot of differences in mold design and methods.
You might take a look at this site. https://rotomolding.org/page/Resources/
Whenever a hard tool (mold, die) is required, changes are costly.
From the videos, it doesn’t require high pressure injection which already save a huge cost.
That is correct. The plastic is in a powder form and it is gravity flow on the inside of the hollow mold as it turns on an X and Y axis in an oven at about 650 degree temp.