Square sails have been used on sailing vessels for centuries, and there is little argument among square-rigger sailors that when the wind is coming from astern a square sail is a fine way to propel a boat of any size, even today.
Anyone with a Bermuda rig knows how tricky it can be keeping your sails full running before the wind, especially in big seas. It is usually necessary to pole the sails out with an assortment of lines and preventers. Even then the person at the helm needs to keep a keen eye on their course to prevent the sails from collapsing and refilling with a loud crack!
Another concern, especially when running with twin headsails poled out, is they cannot be easily reefed. If conditions change, the crew will inevitably have to go forward to deal with the situation, more often than not at the exact same time nobody wants to do so. A triangular Bermuda headsail is also far from the best shape for holding the wind on a run, both in terms of projected area and efficiency.
By contrast, with a square sail correctly braced, these shenanigans simply disappear. Your heading can vary widely, by as much as 30 to 40 degrees, as there is little concern about broaching. The person at the helm or the autopilot will also have little difficulty maintaining a steady course, and the boat will roll less.
Of course, the major disadvantage of a square sail is that whopping great sheet of canvas billowing from a long yard high up the mast: that and the attendant problem of unfurling and stowing, not to mention reefing the darn thing! This in turn precludes the use of square sails on anything but large vessels with large crews: like sail-training ships, aboard which you have plenty of young people willing and able to scale the ratlines and edge out along flimsy footropes to reef
