The type itself is pretty good, with a few good immunities (although obviously not on par with Construct and Undead), even if it has no proficiency, but the Hit Dice themselves are really bad, giving d8 HP, 3/4 BAB, 2+int skill points and only one good save. For a beatstick, the only worse hit dice are the 1/2 BAB ones. Even Humanoid is probably better, since Reflex is more useful than Fortitude when you have so much Con. That's what I meant by "Plant RHD are really not good".
Also, vulnerability to fire? No type has any real vulnerability, and only the [Cold] subtype has vulnerability to fire.