Tire inserts are a popular way to allow riders to run lower pressure in their tires. Lower pressure can improve ride quality and traction under many circumstances.
While Cushcore™ can reduce direct impacts to the rim, it is still possible to experience rim failure due to excessively low tire pressure or extreme impacts. So while rim inserts may help protect your rim, they can't prevent rim failure entirely. Riders should be particularly careful with tire pressure when running inserts which provide tire sidewall support such as Cushcore™. The added sidewall support allows riders to run dramatically lower pressures without experiencing casing roll when cornering. Since casing roll is one of the most obvious indications that tire pressure is too low, this can lead to riders running pressures much lower than suggested and subsequent rim failures.
e*thirteen has not evaluated the effectiveness of any particular rim inserts. Our advice is to set your tire pressure appropriately for your weight, riding style, and conditions. Adequate tire pressure is the best way to prevent rim damage, inserts or not!
For our thoughts on how to set tire pressure, check out this article: What tire pressure should I run?
Running an insert, or not, will not affect your wheel warranty.