I do not have major issues with Marantz remote app, apart from once in a while loses connection with the receiver but I think this is due to the receiver loses some synchronization with the network, not due to the app since as long as I unplug it from power and plug it back again its all good. I would say it works for me 95%. It is very easy and convenient in use, it just don't have that extensive functionality. One thing it is annoying is after the latest update, now every time you close the application (or your phone goes power saving) you need to connect to the receiver again to use it again. Before last update it remembered your receiver, I can't understand why they changed that!
Airplay also works pretty ok for me, sometimes there are issues with the connectivity but it is generally ok. Never really dropped during playback, sometimes just happens to take some time to connect. I have noticed that AirPlay on my Apple TV 4K is much more reliable, maybe that's because it is AirPlay 2.
Can you use MusicCast to stream music from your computer like AirPlay or it needs a mobile app? Most of my music and especially the high resolution files are not located on my iOS devices but on hard disks attached to my computer.