Battlefury is always good for farm acceleration. Cleave, regen sustain, and enough mana regen to spam torrent in the jungles. Getting one should be a function of how well the match is going, rather than an automatic purchase. If you can't get it in under like...15 minutes or so, it starts to become "late". That is, it begins to delay your fighting item pickups like crit.
AC is good on Kunkka because while it doesn't proc his passive...he's still a carry. You still want to be in position to do as many autos in a fight as you can. If you're only looking to right-click when your passive is up, you're only autoing once every 4 seconds. That's not good enough.
Also remember: armor reduction is going to apply to everyone in the aura radius (enemy and ally). For offensive purposes, it means that the Tidebringer splash damage you're doing is going to get the benefit of reduced armor for all targets in range to be splashed. That means more damage on splash in addition to you being able to tank better so you don't have to run around like a bitch in team fights instead of auto-attacking like the man your team needs you to be. Being able to tank better is why Vanguard is a situational item and Heart is a common purchase late-game. You need to be able to tank as much as possible while auto'ing.
Heart is good as well, but they do not fill the same purpose in the same way. AC is best when against a team of auto-attackers where armor is critical and when you have another right-clicker on your team; heart better against a lot of disable and magic damage. Ideally, I'll have both because the Heart's health regen is too good, and the AC's offensive and defensive aura is too useful.
Double crit is not a replacement for AC. It simply depends on how your game is unfolding. Double crit won't mean much if you're getting fuckin murdered, right? If you're far enough ahead to rush double crit, the game is probably already won. If the game is in doubt and you don't have any tank items on Kunkka, you might unwittingly be making the game even harder.
My preferred 6-slotted Kunkka is something like Phase (Travels) + Daedalus + Daedalus + AC + Heart + BKB. Where I would have bought and later sold Battlefury and Shadowblade along the way. Games don't usually last that long, though. Most of my Kunkka matches have been under 45 minutes. The semi-recent exceptions are:
http://www.dotabuff.com/matches/193764429
http://www.dotabuff.com/matches/239260227
http://www.dotabuff.com/matches/629826056
and last outting
http://www.dotabuff.com/matches/826962534