This far north I can't leave plants in pots over the winter, they have to go in the ground. We do stay cooler then areas further south during the summer so I'm sure that helps. I'm also trying to plant them where they won't get baked by the sun all day. Both sides of the garden are shaded now that the surrounding trees have grown taller. This gives them either afternoon or morning shade. The one I did loose was in an area that had more sun than the others last year.
Good luck with the heucherella! I'm not sure I've seen them available at the local nursery...I'll have to look