The thing with solo runs is that there’s extremely little room for error and once you go down then you’re dead (unless you’re COG Gear). The very good players could pull it off on higher difficulties consistently to make it worthwhile. And even if you’re good enough to pull it off, the question remains whether you can do it with the characters you need cards for? I mean, I’ve done solo runs on Inconceivable before but only using Cole, Lizzie, Clayton, Mac, Emile and Lahni. Keegan doesn’t have any health, defence or stim cards so you’d need to be extra careful.
And you get the same number of cards playing in a team anyway, so I’d say go with a team where possible. Even if your teammates are not great, you’d hope they would be able to at least revive you if you went down.
The only time where it may be beneficial to not have teammates is if you’re looking to farm ribbons or certain types of it(Hail Mary, since you need a Salvo, Boomshot or Lancer GL for it, Hat Trick for successive headshots, Methodical from executions). It certainly is more practical to have a team.
Although sometimes I personally simply couldn’t be bothered to find randoms who had a clue of things to do. Which lead to me developing somewhat of a solo card farming strategy on The Line with either Mac or Lizzie that could be run relatively easily on Insane(health and damage off) in 8-10 minutes. Even if it was just one act I would rarely fail and it wasn’t actually that boring. Also good for Torque Bow and Dropshot practice as there is the potential to one shot Scions with a Drop headshot(if RNG doesn’t roll a godforsaken low damage hit on the weapon, a ridiculous concept that should be gotten rid of asap) and a guarantee to take every single head off in one Torque headshot regardless of the damage boost applied to it.