From what I understand, and my limited experience, chems can only be interrupted by rebuilds. As long as you don't have to stand rebuilding with the spanner there is very little chance of anything catching fire, only exception maybe the squid because, really, who needs two main engines Cap? when something catches fire 90% of the times is because the engi is focused on firing and the situation allows for a couple of stacks.
Doing the math, you can pull +/- 500 hp in chem time on most ships, not considering spanner hits but only mallet hence it takes more than 3 banshees focused on a single component to take anything down.