Why "bukan" behind "adalah"?
I think because "bukan" is negating a noun, "satuan" (unit)
Should be “A kilogram...” or “Kilograms are not the right units to measure my feelings”
Or it could even be "the kilogram." I agree that it should be "feelings," if the statement is general, though it could be "feeling," if we had been discussing a particular feeling, e.g. my feeling of anticipation at the next Indonesian lesson.
Why 'bukan' after 'adalah'? (negative marker vs affirmative)
would “kilogram tidak adalah satuan yang benar” etc. be correct ?
no, for a noun (the unit) you must use 'bukan'
Shouldn't just bukan be enough?