The downside is that War Mode will make you vulnerable to other players, which means you might be killed more often. Whether it’s worthwhile will depend on the zones you’re visiting and how active the other faction is on your server cluster. Heavily populated realms usually mean more players on both sides, and more conflict, but on quieter realms, you may rarely run across an enemy.
From my understanding, the world boss is always there when your faction controls the Warfront. Like other world bosses you can only get gear from him once a week. There are other world bosses in the zones outside the warfront and 1 spawns a week just like Legion. I’m not 100% sure that the warfront will be available on launch but I have yet to read anything that says otherwise. The quest also reset once a week as well. Think of Warfronts like old Tol-Barad
Something to keep in mind as well is that this will make enchanting materials more valuable, since it is likely most players will now simply scrap their boe greens instead of getting them disenchanted. Also redundant dungeon/raid gear can be scrapped rather than traded to the (guild) enchanter in the run. Less stuff gets disenchanted, meaning there will be less ecnhanting materials, meaning their value will rise. All the more reason to DE rather than scrap as an ecnhanter.