I responded to a Mason Bee producer who stated that Mason Bees where superior pollinators. He is part right, but the pollination issue is far more complex.
My answer is such.
I provide nests for mason bees as well as bumbles but I must disagree with the blanket statement (pollinate better.) That may well depend on the specie of honeybee they might compete with. Some types like Carniolans and Caucasians are usually late starters, but if you have a mild climate Italian types come out much earlier than Masons. I live in a cold wet area of the PNW and I rarely see activity from the Masons until the season is well along. Bumble bees often are the earliest of pollinators. Best pollinators also depends upon what crop you speak of.
Caucasians have long tongues and can pollinate blueberries, others do not. Masons are preferable. Pollination of apple trees also is more effective with Masons as they stick to the job and pollinate each flower more effictively.
Bees often flit from one to another and sometimes produce substandard small fruit. An orchadist may well prefer to keep them OUT of their orchard and rely on Masons and Bumblebees. Pollination is a complicated story. And you should also mention the upkeep of Masons and that they too if unattended will probably develop a mite problem as do bees. New clean nests are needed every year or two. BTW, I would not count on Masons pollinating early plums!!! Bees do a better job.