Any particular reason as to why vowel harmony doesn't apply to the two suffixes in ferfiakat?
Well... When a word ends with a vowel other than a, e and i, you simply add -k to form a plural. If a word ends in -a or -e, you replace it with -ák or -ék, respectively. Words ending in -i do not have such a precise rule; férfi gets -ak and becomes férfiak in plural. The other suffixes are then added to férfiak in correspondence with the rules, so you get férfiakat for accusative.