Left-handedness isn’t good for you — it’s “associated with fitness-lowering traits” — but being a southpaw yields a certain advantage in many sports — and in fighting, which is why it has persisted so long, the evolutionary story goes.
So, researchers decided to look at left-handedness in the Ultimate Fighting Championship:
The finding that left-handers are overrepresented in many combat sports is interpreted as evidence for this hypothesis. However, few studies have examined sports that show good similarity with realistic fights and analysed winning chances in relation to handedness of both fighters. We examined both, in a sample of the Ultimate Fighting Championship (UFC), a fierce fighting sport hardly constrained by rules. Left-handers were strongly overrepresented as compared to the general male population but no advantage for left-handers when facing right-handers was found, providing only partial evidence for the fighting hypothesis.
I would say that finding left-handers overrepresented is rather strong evidence that being left-handed is helpful, since anyone fighting competitively is in the top fraction of one percent of fighters.
Conversely, not winning more often suggests very little, once the pool of competitors has already been selected for fighting ability — and matched up by fighting ability.