Let's not go completely overboard (like we did for Trubbish) just yet. Walling key Pokemon is wonderful, but having used Chespin, I've found that to wall the Pokemon it's trying to wall, it often has to spend a great deal of its time recovering. This normally wouldn't be too bad, but the issue here is that Synthesis only has 8 PP, meaning Chespin is easily stalled out. One Pokemon this is particularly problematic against is Clear Smog Foongus.
0 SpA Foongus Clear Smog vs. 68 HP / 236+ SpD Eviolite Chespin: 8-12 (34.7 - 52.1%) -- 0.4% chance to 2HKO
(8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 12)
108 Atk Chespin Payback (50 BP) vs. 124 HP / 160+ Def Eviolite Foongus: 3-4 (12 - 16%) -- possible 7HKO
(3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4)
Chespin is forced to use Synthesis at least once every two attacks, and usually only pulls off an 8HKO, which is the same amount of PP as synthesis; unlike Chespin, Foongus has Regenerator, meaning it will eventually be able to overtake Chespin quite easily.
In addition, several of the Pokemon it is purported to beat actually give it a fair amount of trouble.
+2 236 SpA Misdreavus Dazzling Gleam vs. 68 HP / 236+ SpD Eviolite Chespin: 10-12 (43.4 - 52.1%) -- 80.9% chance to 2HKO after Stealth Rock
Berry Juice Misdreavus is actually able to avoid the OHKO from Chespin's Payback.
108 Atk Chespin Payback (100 BP) vs. 36 HP / 0 Def Misdreavus: 18-22 (78.2 - 95.6%) -- 6.3% chance to OHKO after Stealth Rock
Not only that, but Misdreavus can burn Chespin to cripple it heavily.
It's certainly a hard counter to Bellsprout, but that's not actually such a big feat if you consider how it loses to Vulpix, Ponyta, Larvesta, Bulbasaur, Oddish, and Archen (the last two use HP Fire instead of Weather Ball), all of which are incredibly common on Sun teams as well.
It also beats most variants of Gastly, although keep in mind that if Chespin does grow in popularity, Gastly could probably opt to just run Sludge Wave instead, which is, on the Life Orb variant, necessary for the guaranteed OHKO on Zigzagoon as well.
With all that being said, Chespin has actually been a lot of fun, and bulky Spikes setters are just really nice in general; being able to beat Drilbur is actually really nice. I just don't feel as if it's on the same level as Pokemon such as Surskit and Bellsprout, and think it would be a better fit in C.