The strongest modern association of mugwort is with dreaming, scrying and the opening of vision, usually as a pillow stuffing, a tea or a smoking herb. This is the claim most worth examining, because it is asserted everywhere and sourced almost nowhere.
What can be documented is thin. The Anglo-Saxon charm credits mugwort with power against poison and infection, not with visions. Culpeper says nothing about dreams. The dream and scrying literature in English is essentially twentieth century, consolidated in the herbal handbooks of the modern magical revival, and it should be described as such rather than projected backwards. That is not an argument that the practice is worthless; it is an argument about its age.
There is one older and more precise link, and it belongs to a different scheme entirely. Agrippa's list of the fifteen Behenian fixed stars pairs each star with a stone and a plant for talismanic use, and the plant assigned to Regulus is mugwort. That is a documented Renaissance correspondence with a named source, and it is far better evidence than the unattributed dream lore, though it says nothing about dreams.
The plant contains thujone, as its close relative wormwood does, in smaller quantity. Thujone is a neurotoxin at sufficient dose. Ordinary culinary and tea quantities of mugwort are not regarded as hazardous, but the plant is not inert, and the traditional caution against use in pregnancy has a real basis in its emmenagogue action.
Mugwort is also a significant cause of hay fever in late summer across Europe, and its pollen cross-reacts with birch and with several foods including celery and carrot in what allergists call the mugwort-celery-spice syndrome. A plant credited with opening the senses is, for a measurable fraction of the population, chiefly an irritant of them.