Unfortunately this type of advice is more frequent these days and the result of people selling things they know nothing about. Do not trust this LHS for any further advise.

The first real indications of insufficient/rotten rubber will be when doing a loop or anytime pulling up from a down line. The airplane will want to uncontrollably tighten the loop or pullup. What happens the leading edge will rise up under load suddenly increasing the angle of attack. If you are lucky they may hold on long enough to land but frequently they do not.

John