That's it really, comparators don't light up when a barrel has either items or a fluid in it, this isn't necessary but it would be a nice addition.

Probably fluid values would make the most sense for them to track, since that is what is more likely to change without user input.

Fluid values would be ideal yes, you can count items using other methods and detect them using other storage containers, so fluid makes sense.

Alternatively, in line with #555, it may make sense to have the barrel output a comparator signal if it is sealed. Other mods should be able to read / check item contents or fluid contents through clever automation.