My only problem with howitzers is their movement rate of 1. Once they attack, they are stuck wherever they are. Left alone, they are dead meat, sitting outside of the city I just took. In a game where it is relatively even, the enemy will use fighters, bombers or railroad carried ground units to kill the howie. This does not happen against clearly inferior foes, but then it does not matter much what modern units you use against musketeers and archers. If you try to protect an exposed howie by stacking Mech Inf (or armor/marine/alpine) with it, it is harder to kill, but a more tempting target. One way that the ai is smart is that it loves to kill stacks of units. Because of this effect, howies are often just glorified cruise missiles. You shoot them once and then they die. So I often skip the howies and use armor, which can move, attack and then enter the conquered city or retreat to a fortress or city. If thwarted by a strong unit behind city walls, I just launch a cruise missile or two.
Ultimately, I think armor is the best unit in the game. It has 5/6 of the howitzers attack power, combined with 5/6 of the mech inf defense power and the best movemnet rate of any attacking unit except alpine (which is faster off road, but much slower on a road).