Warmoth uses polyester for the base to level out the wood grain, then a polyurethane finish over that for the finish coat.
It's a good finish, very attractive and much more durable than lacquer. It's also not any thicker than lacquer and has no effect on the tone of electric guitars. That's an old myth that got started from the acoustic side of the industry, where finish does make a difference. Regardless of the material used, if it's too thick it has a deadening effect on the soundboard. You don't have a soundboard on electrics, you have pickups that are excited by the strings.
Lacquer continues to be the choice of the DIY crowd because it's just too much trouble and expense to deal with the urethanes. It's also much easier to recover from mistakes or to make repairs if necessary. The 'thanes are almost impossible to repair seamlessly. You often end up doing a complete refinish if it's important that scratches or blemishes disappear.