Define "safe".
Do you mean real world, thousands of people have done it and are now doing it and there have been very few injuries or deaths?
Or, do you mean safe according to the "Laws of Physics"?
I towed a Jeep behind a 1994 Dodge 3500 dually 4X2 with an 11.5 foot Caribou fully self contained camper on it, using a home made 41 inch hitch extension, and no aux braking, for several thousand miles. No accidents, incidents, or occurrences. Nobody was injured or died, nothing bent or broke.
Very few states actually require aux braking for a towed motor vehicle, although many have braking performance standards. Such as, you must be able to stop within XX feet from YY MPH. For the State of Montana, see MCA 61-9-312, Performance Ability of Brakes. California and some other states, and the FMVSS have similar standards. Considering the current "nanny state" politicians at all levels of government, if it was a real safety issue I do believe they would have passed a law or two requiring such braking systems, and setting standards such systems must meet. After all, if a 2 liter bottle of cola can't be delivered with a family size pizza, how could they not have a law about toad braking!
I have never seen an exclusion in an insurance policy which said that the company can refuse to pay if you do something silly, or foolish, or stupid, or illegal. I would be very interested in seeing a case where such a thing happened, or seeing the policy language which would allow it.
IMO, the Ready Brake or the Ready Brute integrated braking system/towbar assembly is the only way to go if you want a braking system. It is simple, mechanical, fairly easy to install, no heavy box to put in and take out, and I have never read a report of a failure resulting in damage or destruction of the towed vehicle brakes.