Well, the transmutation part of zeno's hatsu makes sense. Giving his nen the shape of a dragon is ultimately transmutation. Also, the first time we saw him fight he did not actually emitted the dragon, he was in contact with it the whole time much like hisoka with his bungee gum or maki with her string. Now, the issue her seems to be that zeno is indeed very proficient with emission. He actually emitted the dragon during the fight with the ants and actually manupulated it to divide into many smaller dragons. Ultimately it is a rather high level of emission to say the least. Perhaps it is something he perfected over the years and does come at a great price though. Zeno is very old though, it is possible he is able to use emission to as good as a degree as possible for a materialization user. Its not like what he did is in itself exceedingly complicated. Giving shape to his dragon should come naturally for him. After that it is a matter of having it maintain its power for a moderate distance so that it can be useful. Even then, there is still the consideration that we have no idea of how much power the dragon loses when being emitted rather than being used in the same manner as against chrollo. For all we know there is a considerable difference.