When you do early warmongering you have to research techs which other civs normally will avoid in the start - the war-oriented lower techs, which boost Production and warfare. Therefore you'll miss the peaceful upper techs, which boost Science, Faith and Food, which are three of the most vital yields for progress in the game.
Now, say you're Assyria, and you've reached Mathematics to get the Siege Tower. You pick out a proximate, weak neighbour for an early target, so you can use the amazing Sapper promotion to its fullest. You capture their Capital, and get to choose a tech you haven't researched, most likely any of the upper techs. If you continue this, you'll eventually be able to mostly research military techs, while still getting access to the vital peaceful civs. This allows you to both conquer new lands and develop your older cities at the same time, which means you'll not stay behind in Science - a problem most early warmongers will suffer. You'll obviously need to research some of the upper techs as well, but not as many as you'd been required had you chosen the Huns/Aztecs/Zulu, since you get free ones.
Now you may argue that the UA is not good for reconquering lost cities, but if you are a competent
Machiavellian warmonger, that should not be a problem, as you should only advance when the enemy is weak, and there are few chances of losing cities, as that means you'll have to invest resources in doing something that halts your victory progress.
I also forgot mentioning the amazing Royal Library, which'll help keeping your army well-trained.
As for Hiawatha, he is arguably the worst leader in the game. Here's his trait:
Firaxis said:
Units move through Forest and Jungle in friendly territory as if they were Roads. These tiles can be used to establish
City Connections upon researching The Wheel. Caravans move along Forest and Jungle as if they were Roads.
This may sound great, but notice that the bonuses only apply to Forest and Jungle in
friendly territory, which halts the usefulness by several miles. Unless you're lucky and start in a forest the size of Mirkwood, you'll most likely only have a few Forest or Jungle tiles within your borders. This means the unit speed boost is immediately made pretty pointless, especially if the forest is scattered. In addition, the text is taken literally, and the movement boost to the units is not upgraded with Railroads, which means you'll have to eventually construct Railroads anyway, rendering that part of the trait practically useless. Also, the City Connecton bonus will mostly force you to make weird road patterns to utilise it properly, and since it only applies for friendly woods, you'll still have to build roads through woods in neutral territory.
But I have no problems with the trade part of the UA, and their unique unit is pretty sweet (although the building is bad, since it doesn't include the possibly most important boost of the Workshop: the 10% Production boost).
All in all, I can ASSUR you that Ashurbanipal has a more powerful UA.
The representativeness and leader choice of the civ is arguably the worst in the game, though...
