I can't say my panniers are ultimately better, but made with different philosophy. I would like to explain my ideas behind design and material choice, this probably also helps to understand benefits.
First, saying "plastic pannier" is as informative as saying "metal pannier" - imagine 3 similar panniers made from aluminum, steel and lead. Their all metal, but everybody understands that they have very different properties.
Same is valid also for plastic. My panniers are made from HDPE (high density polyethylene), while most OEM panniers are made from ABS (acrylonitrile butadiene) - easy to manufacture, stiff and cheap. One important difference between these two materials is elongation at break. When ABS elongation is in 5-25% range, HDPE breaks usually at 100-300% elongation. It means for user, that pannier made from ABS breaks after relatively minor deformation, while you can completely fold HDPE and it still doesn't break - check my crashtest video: https://youtu.be/Bc38U5kVgkA.
But main difference between my and other plastic panniers is different philosophy. While other manufacturers have tried to make panniers stiff enough to hold shape (reason to use ABS), I focused to using plastic main advantage over metal - low stiffness and good flexibility - and designed my panniers to benefit from this. And this makes them very different from all others. This also allows me to make my panniers twice as light as common plastic pannier.
Biggest disadvantage for OEM panniers is, that most have only 3 point fixing system - 2 upper and third close to passenger footrests, making a triangle. No support for rear lower edge. This means, that even minor crash creates enormous loads to fixing system and while OEM panniers have rather stiff plastic, they break.
And finally, making panniers flexible means, that they act like airbags, decreasing dynamic forces and protect your bike. I highsided my KTM 950SE at 80km/h and didn't even bend rear subframe - but right pannier upper corner was deformed after crash.
I hope this explains why choose Tark panniers over others