Personally, I might be tempted to pull it home without doing the brake work. You did ask what could go wrong, though. You could be going down a long hill, overheat the brakes on your truck and boil its brake fluid. This would make it much harder to stop. Now it's only really is a problem if you get in an accident. If you do, after all the lawsuits, you might find it was cheaper to fix the brakes in the first place
If the brakes are not working (indicating a lack of maintenance), I would be more worried about the state of the wheel bearings as you could lose a wheel if one set up.
My personal story, along these lines, happened when we bought a used power boat about 10 years ago. I was towing it home (a 15-20 mile drive) when I saw one of the trailer wheels pass me. It shot across the road and took out a Honda that was going the other way. Luckily, no one was hurt. The boat and trailer finished the trip home on a flat bed. Who'd a thought that the previous owner would have taken the wheels off to paint them, and never got the lugs back on tight enough. The bearings were shot as well. Recently, there was an accident not too far from me, where a boat trailer, being pulled home from winter storage, lost a wheel. The wheel went through the windshield of another car and killed the driver. This was a sad reminder of how lucky I was.
The next boat we bought didn't move until I repacked the bearings and torqued the lugs in the owners driveway (in a rain storm).
So you could say I'm a little paranoid about these things. Your call on your new used trailer, though
