I don't really see a use for Feudalism, because cities will grow beyond 6 eventually, crippling your money due to unit support.
It can be an excellent war government. You just need to make sure you continue to expand; your new towns will provide the unit support.
I've only one experience with it, though, in the first succession game I played: SGFN05; India stomps the World. This was a conquest game, non oscillating war, to be exact. We had already researched Republic, but didn't switch, because the unit support in Republic would have crippled us. Then we researched Feudalism, I suppose because it's a high value tech, or perhaps for military reasons; it wasn't my decision.
We were still in Despotism, though, and then I suggested we went for Feudalism as our government. Both Republic and Monarchy didn't look good, but Feudalism would pay for all our troops, and I figured if we would continue to expand and put in or conquer new towns, those new towns would support our units, while allowing our core towns to grow to size 12, the way you want them to.
The team wasn't averse to trying something new, and we were Religious, so we could easily switch back if needed, so we switched. Feudalism worked out better for us than I had expected. We ended up a staggering amount of units
under our unit support, war weariness didn't prove to be a problem, and we never switched again in that game. Feudalism simply was the best option in this game, and we should never even have researched Republic.