Airstream has used torsion axles.
The advantage is that each wheel is independent of the others.
With a single axle, if you have a flat or other failure, such as a bearing that went out, or a bent wheel, you are "done", unless you have a spare or other repair parts with you.
Also with the older Bambi and Caravel
models, there is a very large "failure" rate, because of the smaller spindles that were used. If you have a spindle failure, again you are done until you can replace the axle.
Tandems offer safer towing conditions than a single axle could ever offer. Likewise a tri-axle trailer offers more safety than a tandem axle.
Additionally, with a tandem, if you had a failure such as outlined above, simply stop, remove that tire and/or wheel assembly, and continue on with your towing. However, slow down to a safer speed until you can get the problem corrected.
You can also actually remove one tire from each side of a tandem trailer, as well as from a different axle, and still be able to tow. Again, slowing down is the key to that safety.
Additionally, pound for pound, a tandem axle braking provides much more stopping power than a single axle.
Also, with todays newer style electric barkes, they offer much more stopping power than electric brakes that were used years ago. The newer styles use oval magnets instead of round magnets. Instead of repairing older round magnet brakes, thought should be given to replacing the entire backing plate assembly, which will increase the stopping power from the old 4000 pounds to the new 5200 pounds per axle.
Accordingly, with todays new style electric brakes, a properly wired, adjusted and working properly, single axle has 5200 pounds of stopping power, a tandem 10,400 pounds and a tri-axle, 15,600 pounds. These weights ratings are for 12 inch brakes. 10" inch brakes have far less stopping power than the 12" brakes
A disc brake equipped trailer with the same axle setup, offers even greater stopping power, requires less maintenance, and does not need any adjusting, so that as the pads wear, the braking power stays the same.
In the case of electric brakes, as the shoes wear, they must be adjusted to compensate for that wear, usually every 3 to 4000 miles.
Obviously, disc brakes, at least with some brands of trailers, will be the "in" thing, of the future.
A few companies, such as Airstream, offer disc brakes on at least some of their larger models.