I'm just using it as a back of the envelope measurement between two paid-for photo enhancement apps. Is there any app considered widely successful that has a relatively low number of reviews?

I would also point out, anecdotally, that I haven't heard of anyone in my social circle using Mixel...not friends, but the people I follow online, on both the tech and photography world. This is not to say that Mixel was a bad app, just that it doesn't seem popular enough to buy for the tech or user base alone.