Sorry pull what, but I disagree. I am myself both an airline instructor and GA instructor and I know many very rounded and competent GA instructors who have never set foot in anything bigger than a medium twin.
I also don't understand the term 'rounded senior instructor' it seems like something that people have invented to fit a purpose.
I do however agree that different experiences can be beneficial but that are not a requirement and in fact Can also have a negative influence on people depending on how they are used/understood.
Hilarious, someone else doesnt understand the term 'rounded' but knows very many 'very rounded' instructors.
Anyone who can get a student through a skill test is 'competent' but that doesnt make you a good instructor