What difficulty level are you on? You're right that this is a great army composition, but if the AI has better tech, larger squads, or higher level, tanks + ranged guarantees nothing.
Also, if 1-2 of your tanks die every battle, the economic cost of this tactic adds up. Against high defense units ( Gilden in particular), archers have little stopping power.
Against mounted melee units, even staff-based ranged units may not get in their damage soon enough.
Moreover, when I have defenseless ranged units, and I run across an enemy army with archers - they sometimes get shredded.
If you're playing challenging or under, time to up the difficulty.