Via's in switch mode power supply
The short answer is, as long as your components are spec'd properly, it will probably work just fine. However, if you ever want to sell the circuit, you're going to need to do some compliance testing.

A 600kHz switching regulator with a poor layout will give you lots of radiated emissions problems when it comes EMC test time, especially in the AM band. The key to limiting the radiated emissions is minimize the switching current loop area. In a buck regulator, that is through the switch, inductor, and output cap. You can make that loop smaller, and control it better if it all remains on a single layer.