The nylon cup in the OEM ball joints is fine for OEM applications because it's a self-lubricating material that lasts a long time with zero maintenance on a stock vehicle. The ball joints are only intended for a vertical load, however, so running big tires spaced out from the steering axis means lots of lateral load on the parts, and the plastic obviously isn't going to hold up to that. I've been running Synergy's powder-forged ball joints since 2012 and they're still tight as a drum, but they require periodic lubing since metal isn't a self-lubricating material. It's no big deal, just something to be aware of.