Exactly… Highest wattage I’d switch using regular cheap $5 timer is probably 150W. When using HPS (or fluorescent tubes) there is a moment (when starting the lamp) when there is electric peak - this usually crosses Amps that cheap timer can handle… Its metal contacts bake together. And then someday it simply refuse to switch off on time - and constant light will stress the plants causing hermaphrodites…
Been there, done that.
Use controller with relay everytime specially for lighting sources. Safety above all…
And one sidenote - my friend has almost burnt out his growroom by using faulty HPS ballast. One of its capacitors melted and started to dripping hot fluid down unfortunately on plastic tubing for drip. Whole growroom started burning! 
The problem was he had hung ballast on the wall instead of putting it on top of something non-flammable (like fireclay).