My experience with "seconds" in any product is that they can be seconds due to many reasons, all beyond the control of the buyer. In roofing then it could be anything that leads to a problem - chemistry, weight, color, depth of penetration of rock, size, too many blemishes in the run etc.. Products can be surplus due a private label order that was renegged on. As an example, we used to see lots of automotive engine parts that were sold by auto dealers that resulted from seconds on the assembly line. Maybe a bolt hole was bad, maybe a bearing too tight, maybe the runout too much, maybe the weight was out of spec on a piston or rod. Maybe someone missed grinding off flashing. Or maybe the part was good and they were in a hurry to end their shift so they tossed it in the not as good bin. Maybe it was sitting on a workbench at shift change and no one knew its history. Any time a manufacturer doesn't want to guarantee the product meets their specs, it becomes a second. That alone does not make it good or bad. It is the real reason something become a second that matters. The buyer won't know that unless the manufacturer tells them.
There is one other thing to consider too. Some manufacturers warranty their product in the sense that they will repair or replace it. The question often becomes who does that service, the manufacturer or the reseller? Some resellers want to make a sale, and never hear about it again - they say call the manufacturer and talk with them. Other resellers will take a discount on the wholesale price and handle any claims themselves. If for example the manufactuer sees an average of 15% in claims, they might offer the reseller a 15% discount if they handle the claims themselves. That relationship is not disclosed to the buyer. And, it is why some companies will help you with the claim, and others will tell you to call the manufacturer on a claim. Selling a product as a second could be a way of getting rid of products that just have a higher warranty claim ratio than normal. The idea is the buyer is willing to trade that discount for accepting the liability themselves for any defects. Thus, there can be no single answer from others whether seconds are a good deal or not. To some it might work out great, to others it might be a disaster. The risk is on the buyer. If someone wants a guarantee a product is good, don't buy a second. If they are willing to take the risk, go for it.