I find that custom units help tremendously. Creating units with certain accessories, for instance, can improve important stats quickly. Of course, in a given game, you may or may not have crystal, or horses/wargs, etc. So by customizing, you increase your range of options based on the resources you have in the game. The preset units don't have a lot of mounted units, for example. So, the default stuff is just not equipped to give you the widest range of choices to make the most of the resources you have.
That said, you can still do fine with the default ones. But creating units opens up the game quite a bit, in my opinion.