But the build cost is jumped up 50% from 20s to 30s for a very early-game unit. And do you really want to attack with a 30s archer equivalent?
The NM does have some versatility, and I would counter-attack with an NM, but man they aren't cheap. Also I hate when a nearby AI has them.
As far as biggest usable advantages I would say the Siphai and War Elephant are the biggest practical improvements that you can use

when they are available. (Edit: Mounted Warrior and Keshik are pretty good, too. I do seem to favor fast units.)