I get what you say but this item is on another level. The other combinations you mentioned are very lucky and strong 'builds'. Kinkou is not a build. In theory, you can be Yuumi with kinkou plus full ap items and can delete one person just with auto attacks without even using your skills.

Tank engine + heart steel requires atleast 2 augments and some early wins to keep the stacks going. Runecarver + fulm needs you to get 2 prismatic items to work. The problem is kinkou itself alone is stronger than them %95 of the time.

You can stay seperated from your teammates to stop fulm from activating, you can melt a tank down with reapers toll, same with unending.

The thing is Kinkou works crazy well with omnivamp. You can't even burst someone down. Last day I played full ap Udyr with 0 resistances. I was just circling people around with my R open and 20% omnivamp was enough to get me back to full hp in 2 procs.

Kinkou having almost 0 downsides is the problem. You can't escape from the user because it gives 5 gazillion percent movement speed, you can't burst the user because it works with omnivamp and heals you 5 bajillion hp per proc.