Basilar/basal skull fracture is what makes helmets dangerous in general, i.e. landing face-first on motorbikes often led to cracked bones at the bottom of the s

Basilar/basal skull fracture is what makes helmets dangerous in general, i.e. landing face-first on motorbikes often led to cracked bones at the bottom of the skull, often fatally. There were reports about increased rates among cyclists when helmets were made mandatory. I consider that pretty serious and not minor. Nothing is without risks it seems.