I think a lot of Flat Structure criticism is actually directed at companies with no structure [1]. Where it works well, the structure also seems to be built with a good understanding of a Manager's duties (which can be distributed) versus the requirements of Leaders and (in private enterprise especially) Entrepreneurs [2].
[1] http://www.wattsnext.com.au/Flat-structures-look-good-on-pap... [2] http://www.shirlawscoaching.co.uk/shirlawsresources/2014/3/1...