Oh, you meant in their debut generation? My starter Serperior in White is decent with Leaf Blade and Coil/Swords Dance, but yeah it gets destroyed in online play,
Yeah, I don't tend to consider how mon perform in competitive because it's like my least favorite aspect of the games and I'm always so late to the online-play party that I rarely get to participate. In base BW Serperior sucks dick until like gym 7-8 when it finally gets Coil/Swords Dance but by that point almost every other grass type you can find outclasses it (Sawsbuck has Horn Leech and a better Attack stat + Normal STAB,
Lilligant has better Special with Quiver Dance + Energy Ball or Petal Dance for easy sweeping,
Whimsicott either has Prankster (with Leech Seed, Tailwind, Powders, etc) for maximum annoyance or Energy Ball for sweeping,
Leavanny has Leaf Blade + bug STAB with Swords Dance and a (much) better Attacking stat,
Ferrothorn is fucking Ferrothorn and outclasses almost everything else
in the game even with Rocky Helmet + Iron Barbs alone) and it's just an overall shitty pick as a result. The only two Grass types I can think of that are as bad as it in Unova's dex are Maractus (a 1% encounter on one route that
still has better attacking stats than Serperior, just a somewhat-worse movepool) and Simisage (a shitty starter clone).
It's also weak to the first gym (as is every starter), the third, and the last three in BW1 and the second, third, sixth, and seventh in BW2. So by the time Serperior starts being even slightly usefu, its utility in boss fights plummets to near-zero (especially in BW2, even if Marlon does help it a little).
Why people continue to parade Meganium around as a symbol of uselessness as opposed to Snivy is beyond me, but I guess most people dislike Meganium's design enough (in comparison) and value earlygame usefulness (which Chikorita has much worse than Snivy does) enough that Meganium has more staying power. At least Meganium gets Reflect and Light Screen to be able to provide some support to the rest of its team...