I think you have mistaken how long X7 has been in the market with how long the X7's beta firmware has been in development. While X7 has already been in the market since late 2015, it was first released with Android 4.4.4 and subsequently all the early bug-fixes and firmware update are meant for 4.4.4. The last firmware that is based on 4.4.4., the FW2.0, is actually stable enough with all the basic features working just fine. The current beta firmware (FW3.0+) is actually based on Android 5.1.1 and it is a completely new firmware when compared to the old 4.4.4. based firmware because: (1) First of, Google made a big change in Android architecture between 4x to 5x. (2) Secondly, to solve some of the issue and limitation of 4.4.4., FiiO has to rewrite the audio driver from the ground up in 5.1.1. Note that in FW2.0 and below, only FiiO music app can take advantage of the DAC's full potential while other app is limited by Google's internal SRC. But in 5.1.1 based FW3.0+, all apps have access to the full potential of the DAC without any limitation from the SRC anymore. (3) Lastly, because all the changes involving Android 4.4.4 to 5.1.1. and the new audio driver, all the previous update and bugfix meant for FW2.0 and below no longer apply to FW3.0+. So basically speaking, X7 running on beta firmware (FW3.0+) as well as X5III are completely new project for FiiO - and it only started since October last year. So realistically speaking, it is almost a brand new, 3 months old new system that still need some TLC before it will be as stable as FW2.0.
I think people start to read about all the recent issue regarding FW3.0+ and start to question why a > 1yrs old DAP still has so much problem, but they forgot that it is all a matter of perspective from a limited POV and not because FiiO isn't doing enough to address issue since X7 was released. In fact, I think people should really give FiiO some credit for moving from the relative safety of the stable FW2.0 to the brand new and buggy FW3.0+ in the name of bring new features and functionality to the user.
Haven't done any extensive test on RFI from cellphone or WiFi, though I didn't notice any from daily use.