6 ARC and 6.5G are very cool cartridges, but I'd call them hobbyist/enthusiast cartridges.
Yes, 6 ARC was allegedly made for some special dudes. I have no idea how much it's been used or how well vetted it is.
Go into any Grendel thread and you'll find recurring questions of which bolt, has system, and magazine is durable and reliable with no hard consensus. You'll also find some comical recommendations of "these mags are great if you only load 10 rounds"