It's... kind of the same problem Snivy has in BW, although it gets some breathing room due to resists not being so stacked through the game, and once it gets Coil, it can actually solo many more opponents than one would think in the second half of the game.
It's not that it can't work, but a one-click win is more popular.
Snivy is SO underrated in the original BW. Let me write a novel about why Snivy is actually the best starter.
Its stats are very unconventional (I prefer
unique, but that's just me), which most people consider a flaw. It might be very fast, but 75/75 offenses are kind of bad, even for in-game. It does have good bulk, with most of that bulk coming from its defenses rather than its HP. This is great actually, because it's the perfect bulk distribution for abusing leech seed. On its own, these stats aren't that impressive, but they work amazingly well with its movepool to form a unique game plan like few other starters have.
Its movepool is pretty shallow, but the options that it does have are really good. It gets growth early, which allows it to sweep plenty of early fights. It later upgrades to coil, which allows it set up on most physical opponents. It gets leech seed, which assists this setup. Its STAB-options are good as well, with leaf tornado being a strong move for how early you get it, and leaf blade is about as good a STAB move as you could ask for. Now, its coverage consists of... return. I mean, it's not great, but it actually has great neutral coverage together with leaf blade at least. It also gets some more situational moves like screens or substitute that can also assist in setup.
So what's the result? Snivy actually has the most impressive gym and rival matchups of the gen 5 starters. It can sweep Chilli and Lenora with growth, though Lenora isn't consistent. It naturally has a good matchup against Clay thanks to its typing. It
can set up coil and sweep Skyla and Brycen, despite the type disadvantage. It can solo Drayden with coil if you deposit the rest of your team, which makes it so you can't be phased out by dragon tail. Serperior also has by far the best late game compared to the other starters. It can set up on both Grimsley and Marshal with the defense boost of coil. It's naturally good against Shauntal's Jellicent and Golurk, and it's also mildly useful as a support Pokemon against Caitlin with leech seed and light screen. Meanwhile, Sammurott is basically completely neutral at the E4, and Emboar is completely shit.
The most impressive thing though, is its matchup against N (in Black at least). It completely walls ZEKROM of all things, so you can set up and sweep N. What other starter sweeps a character who is effectively the champion without extreme item support? It's insane.
I also think Serperior has one of the best designs out there. It is so different from any other Pokemon, which makes it really stand out. Its unique appearance fits its unique statline very well.
So in summary, Serperior's good bulk and access to useful support moves allow it to set up growth or coil to offset its rather low attack stat. Once fully set up, it can use its natural speed to sweep through the opponents team. This unique playstyle lets it sweep some really difficult fights, including the champion. Maybe it's just me, but soloing major matchups is extremely valuable to me, more than enough to make up for its lackluster route clearing abilities. Snivy might not be the most effecient Pokemon, in terms of how fast it can defeat most opponents, but it's better than the other starters in terms of effectiveness.
