Braking is one reason why I wouldn't use the Brommie as a tug unless I really needed the folding element and the load is fairly light - Brommies aren't exactly renowned for their powerful braking.
A trombone and a bag of other gear is fine, especially in a flat city centre. For a heavier load or a hillier location, a heavier bike with pokier brakes and lower gears would be much better.
For heavy loads generally, consider the hitch carefully. Many hitch arrangements allow flexing, either in the hitch itself or in the arm between the hitch and the trailer. This makes the trailer waggle unless you pedal verysmoothly, and can also allow it to shunt back and forth when you brake or accelerate. With a big load on, this shunting can end up with your back wheel sliding a few inches with the brake locked as you come to a stop. If it's a straightforward shunt, you slide forward. If it's a combined shunt and waggle, the trailer can push the back wheel out from underneath you, which is distinctly discombobulating.
A stiffer hitch and towing arm helps prevent this shunting and waggling from starting and makes for a smoother and more comfortable experience.