2 issues at play here:
1. How much force does the router generate upwards? Most board cutting involves horizontal forces. When cutting horizontally with a typical spiral bit, the router is pulled down. Big upwards forces are generated when trying to plunge a big diameter cutter. Some cutters will refuse to plunge and then the gantry is lifted off the rails. We happily plunge 40mm [1.5"] ball nose cutters into hardwood without lifting problems. Do bear in mind that spindles and routers do not like big axial loads - one has to limit vertical loads whenever possible.
2. What counteracts the upwards force? The weight obviously. Plus the springloaded motors pushing up under the racks. Plus the dust collecter sucking down sometimes.