Waaaay to many weaknesses.

It's weak to less moves than your normal Grass type.
Although I suppose having a weakness to Psychic isn't as nice in Gen I when Alakazam seems to be the special attacker of choice.

I'm comparing it to the other starters, who both have only two as opposed to four.

IT'S NOT THE WEAKNESSES THAT MATTER
IT'S HOW YOU USE IT

man I really have no idea, I never play competitively

I have almost no use for grass pokemon because there is nothing they can do that water cannot aside from 2x-ing water and having more weaknesses. I normally don't pick any grass types to join my team aside from Victreebell because it is one of my favorite pokemon.

Grass-type moves are badass though, like leaf storm and solar beam. Also, grass-type attacks do double damage against three types (water, ground, and rock). This, coupled with Venusaur's awesome Sp.Atk stat, makes it quite the power house.

I've only chosen a water-type and a fire-type starter pokemon once each, all other times have been grass-type, including Bulbasaur.