when putting plastic fittings into the brass thread of the tank make sure you use some ptfe tape and make sure you dont cross thread or over tighten them, as plastic will thread or crack easily causing leaks.
being a plumber, one other reason not to use plastic is incase you drop something on it it will break and crack off, leaving you with a van full of water and an empty tank, plastic over time wears and can crack as brass fittings wont. not ideal if you use warm water in the tank as this will cause the plastic to expand and contract which can also lead to leaks.