An hour on the machine consumes far less real time than an hour in the boat. Training in a single scull, I have only an approximate idea of how far I have rowed or how fast I am going. But the machine's electronic performance monitor spews out exact measurements. Outdoor times are less meaningful. Although rowers enjoy talking about their times over given distances and course records at certain regattas, everyone takes these numbers with a grain of salt, because wind, weather, current, and a host of other factors invariably influence the times recorded.
Balance is irrelevant indoors, because the machine sits on solid ground.
Hurtling my mass toward the stern will generate momentum that counteracts forward progress. The ergometer, too, has a sliding seat, but, there, the only thing I am moving is digits on a monitor, so I can roar up the slide with abandon. While I want to apply maximum force to the oar, my body weight also represents most of the load in the boat and can become ballast. In contrast, rowing machines exact no penalty for bulk. The most important difference between boats and machines may, in fact, be psychological.