The reason to apply undercoating in the wheel wells is to prevent damage to the fibreglass from rocks kicked up by the tires. You want something that goes on moderately thick and stays compliant. If it hardens, then it won't soften the blows. Thus the recommendation for 3M Rubberized Undercoating. It works well.
In the engine compartment, undercoating is unnecessary and would just add unneeded weight. I happily used semi-gloss black paint.