Follow the electricity from the panel to the bulb. Anywhere along the path is danger for fire.
Bad/low quality circuit breaker? There's a risk. If you ghetto-run romex to your lighting controller/timer or you use extension cords (braided), then there's risk if any of the cords get caught, snagged, pinched, burned, etc. Ballasts usually don't fail, but when they do, the caps will go "pop" and smoke will come out while still pulling power. That's why you need a good circuit breaker. From the ballast to the reflector/socket: same risks as running romex or extension cords. And finally, the bulb. Bulbs can, will, and have exploded. Use good quality bulbs: my ushios and horitluxes have never exploded on me (I have used over 100+). However, I have had bad experiences with Digilux and Plantmax. If a bulb explodes or fails, it will either cause an electricity arc, or the bulb will not fire which causes the caps in the ballasts to pop, which will cause the breaker to go off.
In other words, you can save money on nutrients, tables, RO versus regular tap water, whatever. However, I never try to save money on your subpanel and all the wiring. I get all my wiring done up to code.