Router on a stick has a significant limitation: all inter-VLAN traffic flows through a single physical link.
Consider VLANs communicating:
Packet leaves VLAN host, travels to switch
Switch sends packet up trunk to router
Router routes packet to VLAN subinterface
Packet travels back down same trunk to switch
Switch delivers to VLAN host
The same link carries traffic in both directions. A Gbps link effectively provides Mbps of inter-VLAN throughput due to this hairpinning. For busy networks, Layer switching is better.