Here's a rundown of just how Bungie didn't bother to test this when it's very simple:
-Reason why explosive rounds cancel out the bonus is because it is counted as a normal hit
-Explosive rounds cancel out the Phatom Gift bonus against EVERY form of enemy, so no extra bullet after three critical hits.
-The Hive Disruptor cancels out the Phatom Gift when used against armored Hive -just like the Black Hammer-
So in order to keep the bonus against most enemy types you gotta go with Hammer Forged or Speed Reload. Bungie, how did you not see this was gonna happen? You had months of overhyping hard mode and showcasing the loot, and you didn't even in the least take off Hive Disruptor for the pistol like you did the Black Hammer.
I know people will say "Oh you could just as easily use the other two bonuses", but the fact that one skill tree bonus cancels out the pistol's main bonus against EVERY enemy type is just... wow Bungie, just wow.
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It's funny though, because they still haven't fixed the Black Hammer.