Are excessively thick safety chains bad?

Actually, 5000 lbs sounds about right. You want some margin for the shock load if you need the chains; the weight of the tongue will fall on the chains and there will be a jerk as the slack is taken up. At work we have to use fall arrest harnesses when climbing communication towers and although the rip-out force of the shock absorbing lanyards is 800 lbs the OSHA required strength of the attachment point on the tower is 5000 lbs.

In the UK we use a thin wire which applies the trailer brakes then breaks. This stops the trailer. The issue with chains is that they can cause the towing vehicle or the trailer or both to flip if it becomes detached. Also as you try to stop the trailer hitch piles into the back of the towing vehicle.

I'm wondering how your brother reached that conclusion. Never heard of that, and I've been pulling trailers since the early 60's.

The only things you need to make sure of is that they aren't too short, or too long and dragging on the ground. They need to be long enough to allow you to turn a sharp turn without binding them.