We've sold about 200 coolant tanks since introducing this product, and we've never had the tank structure, or welds, leak.
The sight tube is a piece of heat resistant plastic tubing, held in place by two compression fittings. If the tube isn't cut to the correct length, one of the compression fittings will not make a solid grab onto the tube, and it may seep coolant. It's not a catastrophic failure, and it's easily fixed with a tube cut to the exact length necessary.
We caught most of these by pressure testing the tanks before they go out, but we've had about a dozen of the 200 or so tanks where it was necessary to send a replacement tube - this was done at no cost to the owner.