The X5 III supports up to AptX from what I can tell. So theoretically the device with the better capability on streaming would be 'better'. In which case you can say yes as the M9 is just a source when used with BT and it supports AptX, AptX HD, LDAC, HWA
I've gotten an official reply from FiiO on the reason for whitelisting.
I'm an embedded engineer and one look at the spec sheet already gave me a hunch. The M9 SoC is not powerful enough to provide a good experience if they give you 'full' Android with Play Store. Next, then you may go, OK, no play store services, how about I install my own APKs? It turns out that is still barely powerful enough depending on what apps you install and this can lead to bad user experience. Thus to keep everything running smoothly, FiiO only whitelists apps they know and tested work well with M9 performance wise (well......working good enough I guess).
"What if an app like Deezer gets updated?"
There is no app store with the M9. Since Deezer is white listed. You can simply download the latest APK onto a micro SD card, put it on the M9 and install it with the File Browser tool to get the latest variant.
No one is going to care enough to 'jailbreak' the M9 and root it. Theres a reason they dont give you the play store + play store services and dont allow un-restricted installation of apps. It uses a SoC built for wearable devices (smart watch).
Only whitelisted apps can be installed. I do not believe TuneIN is a whitelisted application. You may talk to FiiO, have them test it themselves and see if they will whitelist it for you.