Well, guess we will never know, because it was not adopted, may be the Malay knows, because they are using it in their special force.
But in my view, the easy adaptation is a plus, considering how many time I was in Iraq doing ranged shooting in 1 minute and then urban warfare in the next, and M4 aren't really good in close combat, most of the time I work on my sidearm rather than M4.
Most don't know this, but P-rail if you shoot enough, they started to come off because of the vibration on each shoot, sometime it comes off 1 millimetre, sometime it comes off 5, sometime it comes completely off, if it happen on base, you can get it fix by going to the armoury, if it comes off during operation. oh well... And it's a hassle every time you try to secure a detachment on it, you need to make sure the groove and the screw line up, otherwise the attachment would get wobbly because you think you tied the screw tight, but since the groove is not lined up, your attachment still have not be able to secure it on .
Other problem such as the gas block blocking your view if you use optics (which guns like 416 or SCAR offer a flip down front site compensated) you need to re-zero your rifle every time you add/remove sight and so on..
There are more, but can't remember them all now.
Most people believe the XM-8 is too German, and don't have the ability (Originally) to use P-rail accessories/STANAG Mag would make the Army to be overly depends on HK for support are the reason why they were rejected.
I am not too sure about that, but that said, having NOT be able to fire it I don't know how it feel against the M4 (the rifle I did fire), so I cannot say for sure, but I tend to believe those were true