I once decided to "punch through" what we call a snow streamer, while enroute VFR. The snow streamer is a line of snowfall which if crossed, may only be a quarter mile through to the other side. In there, it's not VFR, on either side, it's usually very good vis and ceiling.
I entered the snow in the mighty C 150, which has standard instrumentation, other than the AI does have a loss of vaccum warning flag. I was in for only a few seconds, and beginning to correct for an indicated roll. I got about 30 degrees over, and things were not right, so I went into partial panel mode to cross check what was happening. As I righted the aircraft with reference to everything other than the AI, the AI was passing through a roll angle exceeding 135 degrees. No vacuum warning flag.
I popped out the other side of the snow in seconds (less than a minute total time in). I came out upright, with the AI showing just about perfectly up side down. It had appeared to be working perfectly at the moment I entered the snow, and quit the instant I referred to it in the snow.
A second AI would have been great, but its a VFR C 150 - it just does not belong IMC, much less hard IFR. The gyro had seized a bearing, and the vaccum was fine (hence no flag).
But, once I confirmed the failure by flying partial panel, and got the plane erect again. the recovery was complete, and prolonged partial panel flight was easily accomplished. Rooting around in a flight bag to dig out the Iphone and start the app for the stanby AI would have taken so long, and been so distracting, that I would have been in the ground vertically by then.
If you have time to get the Iphone out, and get it going, it's probably 'cause you got the copilot to fly, and they referred to their instruments, so you were all set with redundency anyway!