Most important is - apply the ordinary redundancy and take at least two independent ones. All applications are based on databases and they have a tendency to crash in Murphys Law timeframe.
Currently, I have SkyDemon (actual primary auxiliary tool), AirnavPro (was my prime, but got worse and worse with version, is due to get thrown out of the cockpit) and Jeppesens Mobile VFR (trial version to see whether is is ready to replace SD and/or ANP) plus JeppFD (which is the most reliable of them all). ANP was quite nice, but maintenance and the hazzles of update tangos revealed that these swiss guys are simply not able to handle long term reliability - some versions crashed frequently, the current one often stops working or the VAC dont pop up - so, for me, dead. SkyDemon is quite nice, but also is not very reliable - if you look at the GPS traces you see drops of the app and also, the VAC database frequently corrupts. Jeppesens newest baby is quite ambitious, but not quite yet there. On my flight yesterday I had two crashes of SD and was unable to relaunch the app. Because I did not load all data for the flight into Jeppesen, I continued with ANP.
So, take at least two to not get into trouble.