Why exactly is remaining a Daimyo better ? I take it that you mean Independent daimyo since otherwise you can't wage war ouside of Japan, but still. Only the bonuses to troops ? I mean as a shogun, I like having unlimited vassals that I don't bother integrating.
10% Morale and 10% Infantry power at empire-level government, while having no extra penalties. Only government form to give military bonuses of that scale, or the thing that makes japanese tags like Oda, Shimazu, Tokugawa, Uesugi, etc. to be equal, or even better than Prussia in stacking military bonuses.
Not better than Shogunate for hardcore blobbing runs, but sure is powerful if you want to yolo-stomp the world while badly outnumbered because of the ungodly military that you can get!
(Also obviously, not that good for getting the Unify-Japan achievement

)