Guess whom else Dunsparce beats in BST? Darumaka and Magnemite. I hope you understand the implication here.
BST shouldn't even be that much of a criterion of evaluating a Pokemon, especially because some stats are a whole lot more important than others, namely offence and speed. Dunsparce has 70 attack and 45 speed compared to Watchdog's 85 attack and 77 speed. Dunsparce's not outspeeding much, whereas Watchdog will go first very often, especially if you catch a Patrat at your earliest convenience.
The 10 Red Shards that could be used for tutoring a punch are given to you free, and if we speak about competition for the elemental punches, there's not exactly a whole lot, especially when Rock Slide nails many of the same weaknesses Ice/Thunderpunch do, and most of the elemental punch candidates do in fact learn Rock Slide. If we're really training a Watchdog, there's little reason not to invest those shards into him (Signal Beam, Bug Bite and Seed Bomb, which also cost red shards, are largely situational and are useful on like 1-2 Pokemon each, who may not even be in play).
Just because your run stopped at Roxie doesn't mean Watchdog fails at the whole game - it's just common sense that a physical attacker won't work well against Whirlipede, and this applies to every single earlygame physical hitters (besides one with a super-effective STAB, who are also probably only 2-3HKOing there). There are plenty of circumstances where outspeeding and hitting with a powerful STAB Return is pretty damn effective.
And I honestly don't see how Sunkern would perform better than Patrat, unless you deprived Patrat of the Frustration/Return TMs (which makes no sense as you can neither miss them nor do they disappear after you use them on something else) and employed some sort of bizarre stall strategy involving Ingrain, HP draining and status moves. Clearly Patrat isn't as 'good' (Sunkern lacks the stats to be half-decent at it too) at stalling but there's no reason why he should be used in that role in the first place.
It's no Stoutland, but it's still a pretty fast normal-type that hits pretty hard; I don't see why you'd have trouble using it earlygame of all possible periods when only about 1 Pokemon is really any better than your starter.