Heatsink works just fine on heavy flak, lumberjack, heavy carronade, mercury, mine launcher, banshee, artemis when aimed to destroy hull and not components and light flak. I mean, it is one of the ammo types with the highest damage per clip!
You could use Heatsink for a gunner on Galleon bottom, Mobula top, Junker bottom, all these places where engineers rarely go or have to leave in order to help repairing something more important. It could even work on Squid in case the gunner is using the front gun, who has got time to chem spray that during kerosening and hydrogen/chute vent?
Heatsink prevents fire from happening and when loaded it even extinguishes 3 stacks of fire. Of course during reload it does not do anything, because nothing is loaded it, wouldn't make sense if it magically prevented fires. This makes it less useful on guns with long reload times such has hwacha, when in combat.
When in combat it all depends on your gunnery skill of what ammo to choice: Is heatsink just fine or do we need burst or heavy clip now? You could win with another ammo type than heatsink, but you might lose your gun if you are not quick enough. And if you decide to load heatsink in, but you lose your gun in the middle of the reload, you were not quick enough to reload heatsink or the engineer was lazy, it depends.
Also, Engineers can't fix everything all the time, that is how ships die. Expect the worst and prepare for that -> Heatsink.