Hard to say. Progressing raid bosses takes more time than running dungeons, but farming raid bosses is much faster than running dungeons. So the more bosses the raiders can kill the more efficient it becomes for them.
And I would assume most raiding guilds will still do 1 dungeon per week (like we already do) since that's the most efficient thing to do. If you're in a raiding guild, finding people to do a dungeon quick and is easy. But if you only do dungeons, finding people to kill raid bosses is not as easy or quick. So it still seems to me that the raiders are at an advantage.
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As far as I know, heroic raid bosses drop heroic loot from the cache, and mythic raid bosses drop mythic loot from the cache.
But comparing it to a 14 is not so clear-cut just yet. This is the start of a new expansion, not the current level of 14s in BfA. You probably won't be able to do 14s without appropriate gear to also kill some mythic bosses. Realistically, it would make sense for M+ to be a bit easier than the mythic raid early on. But raiders can do M+, they are not at a disadvantage anyway.