Elder Scrolls Online is an incredibly schizophrenic game
Elder Scrolls Online somehow manages to be both an enjoyable and frustrating experience. Its schizophrenic nature is actually quite interesting once you dive into it, especially considering some of the features that make the game so good also harm its appeal.
The biggest example is the game's use of phasing. A common criticism when it comes to MMOs is that the actions we perform have very little visible impact on the world around us. Sure, we saved that village from cetain danger, but it's still under attack; sure glad I helped do absolutely nothing! In Elder Scrolls Online, though, my actions and decisions are having impacts around Tamriel. That city I just saved now has merchants and vendors. Those people I helped out aren't attacking me anymore. The problem with this, though, is that I'm in an incredibly different phase than anyone who hasn't gone through the same actions and decisions that I have.
The result of this is an absolute nightmare for people who want to play together. Even if you're questing simulatneously, the system isn't quite perfect. The nature of the MMO beast, especially at launch, is that if it could be buggy, it is. Invisible party members is not only a thing, but a thing that happens far too often. Honestly, though, that's to be expected if they slaughtered the group of people I saved.
It's not to be expected in a dungeon, though. As a solitary player, I can live with the grouping issues in the overworld. Yet when I have to waste time to ensure my party members can all work together in a private dungeon, that's just not cool. Because of this,it's not hard to see the argument for "why is this an online game when I have issues playing with friends?"
To be pefectly honest, the best answer I have for that is "because dungeons and PVP are a beautifully chaotic wonder. You just have to be patient." I'm indeed being patient and ejoying myself with Elder Scrolls Online, but that doesn't mean I'm not being annoyed either.