It's even better than a PBS special. WHY NOT You may arrive at your destination at an ungodly time of the night. You may at some point have to use the train restrooms. You will be forced to stay on the most beaten part of the beaten path. Lovely little seaside towns like Sa Huynth, for example, are not on the express route and therefore completely overlooked by most train travelers.
Theft is occasionally a problem.
It is significantly more expensive than most other forms of transport in Vietnam. The heavy window grates are supposed to be down whenever the train is moving. In the beginning I ignored this rule and pulled them up whenever I wanted to look out the window. Since you may be sleeping on your journey, make sure your stuff is well tied down and in an awkward place for a thief to get at. I went back for it within minutes of realizing it was unattended.