Not really, just "ugly". I know plenty of rack mount systems run on a table for years
As long as the box is not depending on a rack mounted cooling fan, tabletop is fine. We do that often in the lab, although mounting them in a rack is cleaner with less cable clutter.
I have a total 16 3.5" HDDs enclosed in 2 Thunderbay IVs, 1 G-Tech Thunderbolt RAID5 enclosure and 1 eSATA OWC RAID5 enclosure sitting atop the rackmount unit
Not sure how many kgs that is