Think with emtb its probably better to look at watts usage with respect to time. Don't know about any others but my rides are nearly always uphills and down them, little time on flats.
One ride I do is 2hr uphill, then 90 mins return so watts/ time and watts/ distance are both dependant on which part of ride you look at.
Overall I find w/ time more useful, the battery after all is always quoted wh....???
I do see w/h can level out on completed ride to give an indication but so can w/time..??
Hard riding I work on 2 and half hrs.
Distance covered totally dependant on terrain.
Slogging up some rutted , stone covered single track will be really expensive for current in respect to distance but with respect to time less so ???