The problem is if you break a cable and have to re-thread it. Happened to me once in 7 years of having an internal cable in the top tube.

If the old cable is intact, you just slide some liner over it before you pull it out, then slide the new cable through the lining - then pull the lining back out.

There are different ways of creating the internal routing on the frame though, and some are better than others when it comes to cable drag.