You can do the Legendary Campaign solo without many issues BUT you'll struggle BIG TIME to run this dungeon solo.
There are convoluted mechanics, timers, and wipes. Add to that a ton of adds spawning non-stop on the boss fights.
So no, it's not even remotely comparable to legendary missions.
I think it's definitely harder when solo, but on par or "easier" when in a fireteam. Either way I enjoyd both the campaign legend solo and the new dungeon in a fireteam. We went in blind with suboptimal loadouts, but it was a lot of fun. I can understand some people want it easier. No problem adding more options like for the campaing were people loved that bungie added a bit more challenging difficulty, because prior to witchqueen campaigns were on vanguard strike difficulty (without judgment, just saying)