What are the main difficulties for someone who is mute to be successful in academia?
I'll open with the obvious answer:

Usually one of the main ways of getting people to know about your work is to give talks about it, in one for or another. It might be possible to mitigate the effect of not giving any talks yourself (eg. by having co-authors doing so, or having a very visible blog), but it would certainly mean taking an unusual approach, at least in my field.