The regen adds challenge to the bosses, a reasonable amount on solo, but it scales a bit out of control with full teams. Definitely beyond what would fit the difficulty curve of the rest of this outbreak, with Pat.
My math could be wrong, but here's what I came up with, based on the information on simplecat's spreadsheet.
Hans vs solo player: about 49 health per second regen
Hans vs 6/6 team: about 231 health per second regen
Pat vs solo player: about 48 health per second regen
Pat vs 6/6 team: about 288 health per second regen
Pat vs 6/64 team: about 168 health per second regen
Pat's running away makes him nearly impossible with a full team. If you don't kill him before he gets away to heal the first time, the team will not have enough ammo for good guns left to have another chance. Possibly, a team could coordinate a scavenging time, letting him regen to full health while everyone fills their ammo with boxes, while not shooting him, and not dying, for truly about a half hour, and then you'd get another chance to dump all that ammo quickly to kill him, or at least knock down another syringe, but that's awfully tedious to even think about.
There's no wiggle room to carry two players with low level or misfitting perks, so with most public teams, there is no hope.
Hans sticks close enough that even when he has great regen against a full team, if your team has enough heavy hitters playing relentlessly, you can break all his shields and kill him without any particular tricks.
On solo, either boss is plenty feasible to beat. AA12 worked for me.
If you want the backpack unlockable and don't want to wait 8 weeks first, play solo, because there's no chance that the regen is going to be rebalanced between now and when the next outbreak starts.