it’s more durable and will last longer.. the plastic ones will break if too much pressure is applied whilst chewing. Not having used the plastic ones, I can’t really comment but the metal ones are comfortable to wear and use.
because the veins go straight down to the valves in the heart and if you have any teeth that are not 100% you can easily get endocarditis, which is an infection of the lining of the heart, typically the heart valves.