I downloaded the app and played with it today in flight. I read the user manual beforehand - it's a bit involved.
Though the app developer tried to think of everything it simply didn't work very well. I tried it on an iPad Mini 2 and an iPad Pro. The altitude was usually off by 150 to 190 feet.
I made a manual correction (part of the app) and then climbed to a new altitude, 1000 feet higher, and the error creeped back up to 70 feet. It was a lot of cockpit wrangling to mess with the correction, set up the app for 'Swipe In, Swipe Out' mode, and interact with it every time I was assigned an altitude change.
As a final data point it was a local flight within a 30nm radius over Central Florida in very static weather conditions. The altimeter setting remained at 30.21 the whole time.
This one looks to be challenged out of the gates, although I'll try it again when I head north in a few days. This concept may simply not be functionally feasible with an iPad or iPhone. If it takes an abnormal amount of pilot interaction for an altitude alert function the app is a liability rather than a benefit.