These are the known malfunctions regarding these two smartphones.
One plus X
These are the problems which users expressed and been discussed in forums.
In announced specifications, it shows the support for GSM / HSPA / LTE but in reality, it supports only LTE bands of 12 and 17. This will lead to some customers will be able to use the only network at HSPA speed.( This will be a major problem in the US).
This problem is also reported by many in forums, OnePlus X is not recognising their microSD card and it has been a problem with many buying the lower internal storage version with an option of the SD card.
One of the solutions is to change the format of SD card to NFTS in format options and if you can't change there, you can do it through command prompt.
This is quite a frustrating problem in this selfie-addicted world. It has difficulty focusing on an object, it also struggles to take photos that aren't blurred. So expecting this problem to solve in next update, if it takes a long duration to solve this then they can expect in the reduction of sales.
One Plus two
Capacitive home button and fingerprint sensor unresponsive
Dual-SIM selection issues
Clear All button from Recent Apps screen has disappeared
Phonebook sync issues when connecting phone to the car via Bluetooth
These issues are explained in detail in above response, you can get over there.