There is no one single designer of the HK416 first off. ANYONE who takes credit for that is either smoking dope or rewriting history. An entire team was involved in its development to include user reps, developers, sales staff, actual designers - a lead and his assistants, prototype makers and testers etc. One of the key folks involved now works at SIG Sauer in NH and is the lead on the development of the SIG516. For that reason it will likely be a VERY GOOD rifle and SIG has some surprises for it that I think give it an important leg up on the current known HK416 designs IF SIG can make it work as intended - All I can say on that.
As with ANY new weapon there will be issues, especially with those run hard like the HK416 has been since it was first fielded in 2004. The units that use them show them no mercy. A few isolated problems have occured but those are in guns with over 60,000 rounds thru them, most of it suppressed and using special ammo. HK has provided fixes to address these rare (< 2) issues out of 1000's fielded. Mostly you are hearing about rehashed internet rumor and comments from the competition wishing their rifle was serving where the HK416 is. They are not. The SIG516 is not. Even the M4 is mostly gone from these units having been replaced by HK416's.
The SIG516 will have a hard time replacing the HK416 but that will depend on many factors. Its AGR is well designed and SIG empoys a good CHF barrel, but it takes more than that. It takes a total package well executed and VERY WELL tested.
The US Army will test all avalable 5.56mm carbines beginning next year. I am sure the SIG and HK guns will be included. It will put up or shut up time and HK416 will be the gun all others are competing against because the very best are using it and they can choose what they want and have chosen the HK416. That makes it the high water mark.