There are many advantages to using trusses, the first being ease of use and installation. The standard rafter takes more effort and work to build onsite than the typical truss system. Also, in certain ways and buildings, the trusses can be much stronger.
One of the disadvantages is making alterations. Because it is engineered, it is actually delicate in certain ways. Any time the plates or framing are adjusted, they should be done/supervised by a qualified engineer.