Another possible confound is that in my opinion metal doesn’t really use even 16ths, there’s a certain “feel” that I guess is a bit similar to swing.
I’m going to have a go at explaining this in text but it probably isn’t going to work.
Let’s use the intro of this: https://www.youtube.com/watch?v=lfpWDOzE5nM
So you’ve got your quarter note “stab” and then your fill in of sixteenths.
In my opinion rather than these being metrically exact the stab is dragged slightly behind the beat and then the sixteenths are maybe a touch faster to make up for it.
We’re talking micro amounts but it lends a certain “gravity” to the first note of the bar and sort of sucks you in.
Obviously at faster tempos the effect will be almost imperceptible but the effect on trying to actually play the stuff is still there.
In a black metal context you get a sort of “fluttering” feel by slight increases of the speed and volume of picking to emphasise certain parts of the riff even though you might just notate it as even tremolo picking.