Personally, I don't think you should run a roots deck without Ellie.
Mildred is a great card imo. I use her more as a dr. I'm prone to win round 3 knowing I'll heal 3 (like arno, somewhat)
willow isn't a good 2* because defeat abilities encourage losing, which opens up too many easy damage options. Maybe Eris with 3 pillz. Easy damage. (Unless you're pulling 5 to win)
Treeman has never been useful imo. He has nothing to me outside of dr immunity. I'd personally choose micken moose or jeto. I'd rather one of them then some lonely 7/4. There's never a good incentive (outside of round 3 / 4, whichever is deciding) to put pills on him. His benefits seem limited to me.
That said if arno was unbanned, arno Mildred would be my to picks then jeto. Treeman, walden and willow aren't worth it to me
The way I see it with Willow is that with her 7/2 stats being a good stat overall makes her eligible to be use on at least trying to block some of the opponents heavy hitter, I mean she can go toe to toe with them and if you win you, you avoided a big damage but if you lose you get a small pseudo-DR effect...
@Soiuku - Rarely though I seen her a couple of times which made her relevant to me.
The point being there are a lot of healing/life Roots cards that generally conflict with her ability.
Though honestly most of which are either banned or rarely used.
Willow is prob better in terms of being a more stable choice.
Drop her round 1 for +3 health in the duration of 3 round.
You are bound to be damaged enough to heal depending on the card your foe drops.
You actually have to win with Mildred and the get/been hurt in-order to actually reap the benefits of the heal.