There might be a few reason why you get no video with ffdshow. You may need to add nextpvr.exe to the application white list in the ffdshow settings or ffdshow will be non-functional.
At the end of day, if you use the same decoders and video renderer as you used in GBPVR, then it should look no different. Both use directshow to do the playback, and how well it works comes down to which directshow components you're using.
perhaps you weren't really using ffdshow with gb-pvr... the fact that you had it selected doesn't necessarily mean it was actually being used. if you look in the gb-pvr logs it should report the actual graph that got used.
server: NextPVR 5.0.7/Win10 2004/64-bit/AMD A6-7400k/hvr-2250 & hvr-1250/Winegard Flatwave antenna/Schedules Direct main client: NextPVR 5.0.7 Desktop Client; LG 50UH5500 WebOS 3.0 TV
the pvrx2-native.exe.log will list the decoders used for playback; iirc it parses the graph after it gets created, and will show if any extra decoders got inserted because a chosen decoder failed to hook up. I admit I'm probably wrong, but this scenario is possible and would explain why you're not getting the same results with npvr.
server: NextPVR 5.0.7/Win10 2004/64-bit/AMD A6-7400k/hvr-2250 & hvr-1250/Winegard Flatwave antenna/Schedules Direct main client: NextPVR 5.0.7 Desktop Client; LG 50UH5500 WebOS 3.0 TV
I'm guessing thats what we were looking for, looks like it's using ffdshow (and like I said, it's displaying/playing properly, smoothly, and proper AR)