By looking at files, debugging on their systems and seeing that certain crashes only manifested on these NVIDIA cards, I'd say.
And not even on price is is close to Photoshop!
Pixelmator costs 15$, while Photoshop costs 500$ (plus taxes).
What were expecting when comparing a 15$ app to a 500$ app?
Don't make a fool of yourself
Pixelmator may have fewer bells and whistles compared to Photoshop - but it's a solid graphics editing app that's priced competitively. For many it'll do (almost) anything they want it to do - at a small fraction of Photshop's price.