Define nice weather. If we are talking winters with temperatures consistently over 0C (32F), little to no snow, seattle and portland are great for winter.

Nice weather for homeless people basically means not strong shelter required to survive, unlike the midwest and east coast which consistently freeze in winter.

SF isn’t a tropical paradise, but temperature extremes that cause death aren’t very common.

But relative to a lot of the US-- Chicago, New York.. it is very survivable.

Definitely, the winter I visited Mountain View it was 2C (35F) and ground frost.