Question about Triac vs Rlay
Relays are as dead simple as it gets. You turn them on and off, they work asynchronously, set it and forget it. They work with AC and DC. 

Everything that makes a relay great makes a triac terrible. Triacs only work with AC. Triacs require support circuitry, for the drive to isolate it from the controller, a snubber if you are running inductive loads and possibly a zero crossing detector. Triacs are synchronous, meaning they (may) require constant babysitting. 

That being said a triac can make up for the short comings of a relay. With a triac you can get a soft start to avoid large inrush currents. You can throttle a load which can sometimes give speed control (AC is weird that way). Also triacs are immune to physical shock where relay contacts can bounce.