bull. Out of all the Grass starters, Venusaur's actually the one with the least weaknesses.
Venusaur: Fire, Ice, Flying, Psychic
Meganium: Fire, Ice, Flying, Poison, Bug
Sceptile: Fire, Ice, Flying, Poison, Bug
Torterra: Fire, Ice (x4), Flying, Bug
As for the dual types, Venusaur loses the resistance to Ground but gains a resistance to Fighting whilst Torterra loses the weakness to Poison but also loses the resistance to Grass and Water. That crippling quad weakness to Ice (usually reserved for Dragon types) also hurts...alot. Typewise, Venusaur's actually the best.