Endless Co-op has big issues that don't apply to Solo.

[adri]

• "Time's Up" curse is gamebreaking.
  If your buddy dies with the curse active you cannot resurrect them. The damage tick breaks the res attempt and your only option to make it is tanking the damage for a full minute until they can self-res and then hope they can reach the door.
  Not only this, but if your buddy dies outside of an open door and you are in the safe zone, you will continue taking damage while inside the safe zone.
  
  
• Stat increases from co-op appear to be non-linear and multiplicative.
  Having played Co-op Endless from G4 to Hard, the HP scaling is completely out of whack compared to both the Solo Endless experience and Co-op campaign. It'd be too complex to exemplify here, but even just Co-op Hard quickly scales mob HP to the point you have to deal multiple tens of thousands of HP to kill some goliaths, even before you're done building your perks in some cases.
  Damage/Aggro range is harder to test, but my impression is that they're also affected because when comparing to equivalent runs in Solo to extended experience in Co-op, Solo seems to have a lot less bursts of damage from mobs.
  
  
• Inability to pause or save and exit even if your buddy leaves.
  You buddy cannot rejoin after disconnecting (which happens much more often in Endless compared to the Campaign), but the game will still continue counting your run as multiplayer and not let you S&E to continue playing Solo later. This makes playing randoms a huge turnoff, because if a player randomly leaves the run can't even be saved for later if you liked your character.
  Also even if you can't S&E, at least let people pause by both pressing Esc at the same time.

There are more issues in terms of player choice of difficulty and gameplay time demands, but these are the most objective ones that I think should be addressed regardless of any balance concerns.

A more personal one that I think should be considered is that multiplayer is a big draw of this game and not being able to use the S&E function can make the mode unappealing because runs take time. A couple of deaths quickly translates to spending 2-3h in reaching the endgame state of your builds, but you might be running out of time irl by then.

Also, I have the inkling that difficulty selection is just broken for Endless based on personal experience. Next time we get around to play I'm probably going to try a run on Tourist or whatever the lowest option was, but from the minimal change I've seen between G4->Hard I'm fully expecting big mobs to still scale to massive amount of HP and their damage/aggro also going over the top.

Just a reminder, this is all based on Co-op which has its own additional multipliers. I've ran Solo and watched runs where it's very viable for minmaxed damage builts to explode big targets in seconds in G4- but while I don't want the same experience for Co-op, something is very wrong with the scaling.