Suggestion 1

In short: make the flint and steel mode actually simulate a flint and steel being used on an object rather than just lighting a fire.

There are quite a few mechanics from other mods that require you to ignite something with a flint and steel, like Eidolon's braziers or Savage and Ravage ominous banner burning, and it feels bad to have something that's literally described as being able to act as a flint and steel that is far more expensive being unable to perform these tasks.

I think this kind of behavior would also be good to apply for things like the Emperor's Chalice, which currently cannot be used for things like, say, filling Eidolon's crucible with water, and it's possible some other specific mod interactions that might require usage of a lava bucket wouldn't work properly with the Infernal Chalice if it works similarly.

Would it be possible to detect whether a given block wants a behavior to happen because of a water/lava bucket/flint-and-steel being used on it, and simulate that more directly? I feel like I've seen some other mods do this.

Suggestion 2:

This might be fixable just by implementing suggestion 1; basically, you can't ignite soul soil or soul sand into soul fire with the Pyromancer's Staff. It just flickers briefly with normal fire and then extinguishes.

Related, you also can't light vanilla candles with the Pyromancer's Staff.