Whereas a dethatcher uses spindly tines (you might call them ‘prongs’) to lift some of the thatch in your lawn, a scarifier has more aggressive blades that tear up deeper-lying debris and much of the grass itself, as well as the thatch.
Scarifiers may be suitable for gardeners who want to tame a lawn that has grown wild, beyond the remedial capabilities of a dethatcher. There’s no doubt that a scarifier can go further than a dethatcher to remove mosses, weeds and thatch – but in the process, they do a lot of damage to the grass and its roots. For routine lawn care, it’s usually best to use a dethatcher; and for last-ditch lawn rescues, consider using a scarifier instead.