I've tested multiple external microphones with multiple configurations on the iphone X and have nothing but problems. Whether I'm using the included 3.5mm to lightning adapter, lightning specific microphones (such as the Shure MV88), or a lightning to USB adapter with USB mics, all of them have issues. Problems range from audio interference, audio glitches, complete dropouts, or no audio at all.
The only company that seems to acknowledge this problem is Shure who claims there is an issue with the newer iphones (Shure on Twitter: "Unfortunately, this is an issue that Apple introduced with the release of the iPhone X, iPhone 8 and …).
Never had any problems with external microphones on my older iphone 6S other than minor interference issues that could be solved by switching to airplane mode but the iphone X has been a totally different story so far. Hope that a future iOS update will solve these problems.