Doesn't it only cyberform organic matter? If it does, then why didn't galvatron instruct KSI to just drop it in a forest or something? You would get way more organic matter and you wouldn't have pick the majority of it out of the myriad apartment spaces that their are in Beijing. You'd also be able to build an army more covertly than just destroying the most densely populated city on planet earth, and I'm assuming that's what galvatron wants. Also if transformium is supposedly what all transformers are made of then why do the KSI bots made of transformium do that weird particle physics transformation when they go from robot mode to vehicle mode?
We saw rocks also being cyberformed, so I guess buildings can, anything can! Tucci said he wanted to detonate it in the desert, deserts usually mean sand and sand is very, very small rocks. As for the KSI bots, well Tucci said he (his team) refined the metal, explored their "geno" so they could 'programme' any form of what they like into it, hence why Galvatron looked the way he did. The bots didn't need to scan a vehicle. By exploring their "geno" they can do with it what the will... I'd say it was a more quick and efficient way of Transforming for built drones anyway!
By setting the seed of in a city, he'd not only be getting the material to make a new army, but he would be also killing all the people who live there too, he's trying to make a statement by doing so. KSI says that their models are better than Transformers themselves, so an updated version to transforming would be understandable, that and there also needs to be a sense of unnatural difference between them to show how strange these man made cons are, they're nothing like Cybertronians have ever seen, they're abominations in a way.
Yeah, Brains explicitly said that Galvatron wanted to detonate the Seed in a huge city to make a huge "I'm back!" statement. Keeping it secret wasn't the objective.
Cause a city is a better background for a Transformers fight than a forest. Also the KSI robots were made purely out of transforium. The Autobot were made by someone else and not from transforium