The mason bees working in cooler weather is definitely a factor in my making the move, but not the only factor. I've lived at my present location over twenty years and have watched the bees on the fruit trees and bushes each year. Until the past couple of years I'd see a lot of honeybees on my apple tree. I'd certainly see less honeybees on cool days but it blooms over such a long period that there would be some days warm enough for honeybees sometime in that period. Maybe just mid-afternoon many days, but I'd definitely see honeybees on the apple tree while it was blooming.
This past year I didn't see a single one. It wasn't just because it was a colder spring though. I have a couple of plum trees I planted a few years ago. They sometimes bloom a week apart so I miss out on cross-pollination and get practically no fruit. This last year, before the first one quit blooming we had some dandy warm weather and the second plum tree had a quick burst and caught up so they were both blooming at the same time. The weather was definitely warm enough for the honeybees to be out.
I didn't see a single honeybee. Not on the apple tree, not on the cherry trees, not on the plum trees.
Loads of honeybees in the late fall coming up to my hummingbird feeder from hives that a beekeeper set up in the summer a half-mile or so away, but none while my fruit trees were blooming. It didn't use to be that way.
I still got some pollination from bumblebees and various other insects, but fruit production was way down from what it used to be.
During cold weather they are contently in motion to keep the hive warm and that burns up a great deal of energy. If you have any animals in the area (including humans) that may have raided the hive and taken too much of the stored honey you can kill off the hive.
Also a lack of early blooming high nectar flowers can kill off hives of wild bees where bees that are cultivated get fed when there is a lack of wild nectar.
I am planting black locust as both a living fence and to help the bees. Now I can add locust borer to the list of pests I have to fret about.
But masons are a great back up bee and while you don't get the bonus of the honey you also don't have to worry about getting stung.