If you are talking about the buildings in the new Alpha Strike box, they are still cardboard, but once assembled they are fairly sturdy. You could put as many plastic mechs as would fit on top of any of them without any issues. Metal minis might cause a problem if you put too many mechs on a single building, but that would only really be the largest buildings where you might have that problem. The medium buildings should be able to support two metal minis without excessive sagging, and you can't fit more than a single mech on the smallest buildings so they aren't a problem. If you did put three or four large metal minis on one of the largest buildings, it might sag, but I don't think it would collapse entirely. I haven't actually tried filling the top of the largest buildings with metal minis though, so I'm guessing on that.
I would say that the buildings were designed with plastic minis in mind, but most of them aren't going to have a problem with one or two metal minis on them either.