With the advent of Garmin Pilot, Foreflight and other flight planning sofeware, it seems that basic flight planning skills have been set aside. For example, during a check ride I will ask; "What will our ground speed will be today?", "What power setting will you use to get your planned true airspeed?"' and "What will our fuel burn be?". Unfortunately, I sometimes get the confused puppy dog look? I'll give you a hint to the answers. All flight planning, regardless of using an app or not, starts with the AFM/POH.
Some airplanes have been highly modified since leaving the factory floor. How have these modifications changed the AFM/POH performance numbers? I once had a 1976 C-172 with an Airplaines 180 Horsepower engine upgrade. If I used the factory AFM/POH performance numbers I would have been off on my ground speed and fuel burn by a lot. I had to use the Airplaines AFM supplement to get correct data. This also meant I could NOT use the generic C-172 performance data found in my flight planning app. I had to modify the app data to get accurate ground speed, fuel burn and ETE.
I personally use Foreflight for most of my flight planning. I don't care who's app you use but each app has a couple of common pitfalls. Like, in order to correct winds aloft data you have to have an internet connection AND have an accurate ETD entered. Without this you will have incorrect GS, ETE, ETA and fuel burn. Another issue. How does your app correct for Deviation and Variation? Something we use to think about using a paper flight plan form is no longer discussed. Is this a big deal? It could be if you're not applying the information correctly.
Don't misunderstand me, I embrace and use technology as much as I can. You just have to understand where the pit falls are to keep you out of trouble.