Agree that there is no simple answer, or even a well-understood definition of "safe to tow". In my opinion, 40-45 mph winds would be very scary. So would 25-30 mph, but Mark & Kim clearly disagree. Much depends on your tow vehicle, your driving skills, and your willingness to endure stress and risk problems.
Smaller trailers and lighter tow vehicles will be more affected by any amount of wind, simply due to their lighter weight. However, big rigs have more "sail area" and get more wind push as a result. Gusts are more difficult to deal with than a steady crosswind because you get pushed too & from, but a steady wind is downright tiresome as you have to constantly pull the steering wheel sideways to counteract. I've driven both trailers and motorhomes in strong crosswinds and it ain't fun, even though it can bemanaged.