It comes very much down to who your adversary is. Eldar with all those starcannons are unmatched by any other army when dealing with marines and chaos. Though we might say Guard have more firepower overall thean Tau, because of their more versitile big guns, if Tau were to fight Guard they should win as they have the majority of AP5 pulse weapons to deal with all the infantry, and the railguns to deal with their tanks. I have seen it done and once those russes are done with the Tau firewarriors slaughter the infantry. KILLS on 2+!
Lets not forget the marines, they can field a nasty array of weaponry too and are much harder than tau and guard, Lascannons, heavy bolters, demolishes and whirlwhinds as well as trust bolter fire combined with that power armour means business. As well as dishing it out you have to be able to take it back remember! NECRONS TOO. Anything with a save of 4+ or worse is in for it. Just about everyinging can fire on the move, and they are even harder than marines. They do however lack AP3 and AP2 weaponry. Making them awasom against guard, but poorer against marines.
I could go on,but the point is each army is more directed to dealing with a certain type of foe.