Hi,
I do sympathise with you about the flyaway that you experienced and I can appreciate the fear that you must feel over the loss of control.
As others have already said the Phantom 4 series is well engineered with built with dual IMUs etc and is renowned for its reliability. That said the systems are not infallible. However, we should all be prepared to look closely at our own actions because pilot error is unfortunately a common factor. Last December I posted an account of my 'flyaway' that was purely my fault and I have cut and pasted that info below:
Over a year ago I nearly lost my P4 when it started to misbehave in flight. After take off I intended to use the course lock function so turned the P4 on to the course heading I wanted but I noticed that the P4 continued to turn slightly after I stopped the stick control and when I turned the bird back onto the desired heading it drifted past again. So although I noted this odd behaviour I didn't fully realise the danger. After I engaged course lock the P4 flew away but was in sight and I managed to fly it back. Luckily I was wearing brown cords. I landed it in a panic and checked over the drone. I did the compass dance and all seemed well so I risked taking off again (yeah, I know!!!) and it flew off out of sight beyond tall trees but I eventually landed 1/4 mile away and recovered it. The first lesson here is to note that you must take heed of any unusual behaviour by the drone and immediately land it to investigate.
I have to put my hand up and admit that this issue was an own goal - totally my fault. After recovering the P4 and while doing the compass dance for the 5th? time I realised that the compass error increased as I approached my P4 back pack. I first moved the pack a few yards to a new position suspecting that there was an object in the ground causing the anomaly and repeated the dance but the compass error persisted. It dawned on me that it was the pack itself that was the problem. I started emptying out the pack until the issue changed. The weather was cold and I had worn a pair of shooting gloves to the take-off site. I had taken these gloves off before flying and put them into the backpack from which I launched the P4. These specialist gloves allow the first finger and thumb to be exposed for operating the trigger/bolt on a gun. These parts of the glove are clipped out of the way by TINY magnets built into the fabric of the gloves. When I found the cause I felt like a complete fool because this silly mistake had nearly cost me my P4.
I thought that it was worth posting this info as a warning to others so that no one else falls in to this trap. I strongly suspect that the same problem has caused many of the reported flyaways. We all need to be aware that tiny neodymium magnets are built in to so many every day items - such as the clips on mobile phone and tablet cases, glasses cases, key fobs etc etc. After this incident I completely emptied out the back pack again at home and focused on a screen shade I had bought, but very rarely used, for the Nexus tablet that I use on the controller. To my horror I realised that the fold-flat screen shade when folded out and mounted on the tablet relied on built in flexi magnetic strips (as used on fridge magnets and shower doors) to hold it in shape. That shade immediately got binned and I made a replacement out of black card.
I then thought back to a non-DJI drone that I owned a year earlier. That drone eventually met its end when it started to fly erratically and uncontrollably. Because it had drifted near to houses and I was not in control so I was forced to initiate a forced motor stop to bring it down. Of course I couldn't remember whether I was wearing the same gloves but it did make me wonder. We live and learn. Fly safe.
All the best, Martin