The softshell category is very broad- with some being fully waterproof and insulated, while others being slightly waterproof and very thin.
Although Patagonia, Buffalo, etc. used softshell ideas proir to Cloudveil, it seems to me that the use of Schoeller 4 way stretch fabrics is what really brought about the current understanding of a softshell.
In winter, at least in the NE, they are fantastic.
Scenario 1: baselayer (your choice), thin midlayer (100 weight fleece or same) and hardshell.
-In below freezing conditions, while wearing a pack and actually moving (hiking, climbing) this setup leaves me with a soaked back, frost build-up in the jacket, and generally wet. Even without the fleece, this occurs.
Scenario 2: baselayer and Schoeller jacket (add a fleece if below 10 degrees).
Same activity- but the breathability of the Schoeller means no frost buildup and much, much less moisture rentention. I stay drier, and the fabric is quite and does not impede mobility. The only concern is that the fabric is about 75% windproof, not ideal for above treeline (my system overcomes this… but it is something to note).
That said, the variety of softshell fabrics is staggering- and some really are just insulated hardshells (fully waterpoof, poor breathability). Still, something like a non-membrane stretch woven is wonderful in winter… I have not used my hardshell jackets in winter for years (well, once at a very, very cold belay, but not while moving).
For summer, many softshells are too heavy for backpacking. However, I'd take the Rab Alpine Pullover or marmot Driclime jacket for climbing over a hardshell or ultralight windshirt.