My wife and I have almost finished our playthrough in split screen, and I've done a act or so solo, so let me share my experience

1. The performance can get a bit spotty - I've got a really nice rig, and admittedly I'm playing on max setting and 4k resolution, but we do have frame spikes in split screen that you don't see in solo

2. The way inventory is managed is kind of a pain, each player has their own inventory, comprised of their characters stuff, as well as whatever NPCs happen to be associated with that player, so it's not as straightforward to manage as in Solo. Also if my wife has something in her camp chest, she has to get it herself, I can't get it for her.

3. The way dialogue is handled is a mess (though I'd argue this is partly a problem for solo mode too). If both players are in separate dialogues at the same time, then only half the audio actually gets played, and which half gets played is super inconsistent, to where neither player is really having a smooth experience. Additionally, which player is in control of a dialogues is whoever triggered it, which wouldn't be so bad except you often wind up triggering them on accident

4. Combat feels slightly slower/more boring, because you're waiting for another person to make their turns, and that only gets worse if they're playing a build that has lots of minions. I often find myself taking my phone out during combat because it gets boring waiting for them to do their turns.

5. Which characters belong to which controllers gets mixed up all the time, and we're always having to fix it when we first hop on

Those gripes aside it is a lot of fun, you just have to learn to deal with the quirks. But it's pretty clear while playing that it was designed as a single player game, with couch coop tacked on top of that primary goal