For bees the structure of flowers is important. The reason being (no pun intended) that different bee species have different tongue lengths.
So for short tongued bees like White tailed bumblebees, simple flowers are often best. In my garden these bees feed on the small oregano flowers, and bramble and I have even seen them feeding on buttercups.
In contrast, Garden Bumblebees have the longest tongues. So they visit plants like runner beans and foxgloves.
Common carder bees lie somewhere between the two and so you will see them feeding on many different flowers – today I even saw one trying to get at a runner bean flower.
Of course bees are clever and some will “rob” nectar. They do this by drilling little holes in the side of the flowers, to get at the nectar (without pollinating the plant!). I have seen them do this with comfrey flowers. Other bees (and different species too) will come along and use the holes too!