D'karr
Adventurer
However, rules can be written in such a way that unforeseen rules interactions, such as the one in the fireball example, can be handled by application of one subsystem to another and/or an extension of guidelines that exist for similar effects.
Once again I point to the opinion that it is generally more time consuming to stop the game in mid-stream while a "rule" is researched, than to adjudicate it on the fly.
Will some adjudications be "more difficult"? Absolutely, but IMO the DMs job is to keep the game moving and the action going. Stopping to look up how Green Fungus Acid interacts with the soles of the characters feet is a waste of time, IMO. Just make a ruling and move on.
In 3e there was nothing more frustrating to me, than to spend 10-15 minutes waiting for a DM to go figure out how X or Y interacted. Or for a player to argue the point and then have to spend time while he researched his point. I'm really glad that nonsense is over.