Took me close to an hour to find all the memory imprints with a spider familiar and a 20% drop pendant so yeah pretty low drop rate and I farmed them from the 2nd map from the end portal which is sorta circular so easy to go around
just kill both firebrands forms, as both forms drops the memory imprints.
dont know why everyone says its very lw drop rate.
i did both quest in about 40 minutes hunting firebrands.
wasent long at all.