I completely agree with Ivanay. I'd also add that underprocessed hair (at least in my case) can take on a frizzy texture. For example, I had a patch of hair that was underprocessed in the middle of my hair shaft. When in rain, humidity, etc. that part always frizzed up tremendously, EVEN when my natural roots did not. It made it hard to detangle and comb through also.
But again, the idea with any chemical process is to have a uniform consistency throughout the strand. Underprocessing indicates that the strand is not uniform. When I texlax, I'm trying to retain some curl all around, since my hair is fine.
Hope that helps.