Having been to Seattle, it's the loudest place I've been two. I think it's a combination of the shape of the stadium, plus it's pretty much always full, and a tradition fans have kept up once they had that reputation. Never been to Arrowhead, but I understand it's pretty loud too. They claim they hold the world record for being the loudest. But from all I can tell from people who have been all over, Seattle is consistently the loudest. Saints, Vikings, Packers all have loud fans.
Arrowhead may be a more intimidating place to play. The place is aging, the owners won't put any money into it, Jon Gruden said there's a lot of concrete heading back to and into the locker rooms, and when you're heading down the tunnel and hear the crowd it's a lot like a penitentiary. Eagles/Lincoln has to be the most hostile place to play though, period. Fans there earned that reputation, and relish it.
With all those endorsements, we should just pencil in Justin Herbert for MVP next year, and probably a couple super bowl wins over the next few years. I'm expecting to see it, and understand these last few years have been an anomaly. Next year we'll finally see the real Justin Herbert, and for a few years after. Sure.