alt Hacker NewsLACK tables specifically are well proven to be quite sturdy actually. They happen to be just the right width for servers / network devices, and so people have used them for that purpose for ages. Search for "LACK rack", or see e.g. https://wiki.eth0.nl/index.php/LackRack. 20kg is nothing; I've personally put >100kg on top.
They're a bit less usable that way now. The legs are basically completely hollow these days so you're not actually able to bear much weight on the screws so the only option is stacking the items so the weight is born by whatever surface is below the "rack" at which point you could just as easily call stacking the equipment an air rack (or an iLackaRack maybe /s).